Invertebrates are animals that have no backbones, vertebrae, or spinal columns as a result lacking internal supporting skeletons. The term invertebrate represents a wide range of animals in the world ranging from single cell protozoa up to the more commonly known insects and crustaceans (like moths and crabs) and including animals such as jellyfish and slugs. More specifically, it includes the tunicates and lancelets of phylum Chordata, as well as all animal phyla other than Chordata. There are eight major invertebrate phyla types, which are described below:

1. The Sponges (Porifera):
   

Animal phylum consisting of the organisms commonly called sponges. It is the only phylum of the animal subkingdom Parazoa and represents the least evolutionarily advanced group of the animal kingdom.

Sponges occur in rivers and streams, from rock pools to the deep ocean depths, from frozen arctic seas to the warm tropical seas. They are perhaps at their most beautiful in tropical marine seas. There are about 10,000 known species and though their basic organisation is simple and remains constant throughout the all species, they do manage to show a great variety of forms.

A sponge is a simple organism that is easy to describe. A sponge is a sedentary, filter-feeding metazoan that has a single layer of flagellated cells that drive a unidirectional current of water through its body. Such a brief description though does not do them justice. Sponges are an ancient and highly successful group of animals. In the Palaeozoic, they are believed to have comprised more than half the biomass in marine reefs. Sponges have been living in the waters of the world for more than 600 million years, and can now be found in all marine and many freshwater habitats.

Here are the major characteristics of Porifera:

• No definite symmetry;
  
• Body multicellular, few tissues, no organs;
  
• Cells and tissues surround a water filled space but there is no true body cavity;
  
• All are sessile, (live attached to something as an adult);
  
• Reproduce sexually or asexually, sexual reproduction can be either gonochoristic or hermaphroditic;
  
• Has no nervous system;
  
• Has a distinct larval stage which is planktonic;
  
• Lives in aquatic environments, mostly marine;
  
• All are filter feeders; and
  
• Often have a skeleton of spicules.
  

Sponges are built up from relatively few cell types, the main ones are classified into the following three classes:

1.1    Class Calcarea (Calcareous Sponges):

Sponges in this class are typified by skeletal spicules composed of calcium carbonate. The spicules often protrude through the epipinecodermal covering of the body wall, giving the organism a rough texture. Calcareous sponges are small, usually only a few inches high, and are generally dull in appearance, although several species are brightly Coloured. Members of this class are among the simplest sponges, and all three morphological types–asconoid, syconoid, and leuconoid are represented. There are approximately 150 known species, exclusively marine and shallow-water dwellers.

Here is an example of calcarea or sponge (Figure: 01):

1.2    Class Hexactine (Glass Sponges):

These are deep-sea sponges (Figure: 02). They lack an epidermal covering, and their skeletons are composed of spicules of silica. The spicules, which often form a latticework, have six points or some multiple thereof. Glass sponges are pale in colour and are cup-or basket-shaped. The spongocoel is large, and the osculum is covered by a grillwork of fused spicules. When the living tissue is removed, the cylindrical skeletons often have the appearance of spun glass. The glass sponge known as Venus’s-flower-basket (Euplectella) supplies a home shrimps that become trapped by the lattice of spicules. The body plan of Hexactinellida is between syconoid and leuconoid;

1.3    Class Demospongiae:

Most sponges belong in this class. It includes sponges with a skeleton made up of silicon-containing spicules or spongin fibers or both. In the latter case, the spongin provides a matrix in which the spicules are embedded. The Demospongiae vary in size from small, encrusting forms to very large, irregular masses. All are leuconoid; many are brightly Coloured. The freshwater sponges (family Spongillidae) belong to this class; they are frequently green because of symbiotic algae that live in the amoebocytes. The fibrous sponges also belong to this class; they include the common bath sponges, Hippospongia communis and Spongia officinalis, and most of the other sponges used commercially. The boring sponges (family Clionidae) are extremely interesting because of their ability to bore into calcareous rocks and mollusk shells. They begin their boring as larvae and spend their lives in the tunnels they form. Sulfur sponges (Cliona species) are bright yellow boring forms inhabiting shallow waters on the east and west coasts of the United States. Here is an example (Figure: 03):

There are three main types of canal system in sponges: The simplest form is Asconoid, here the canals run straight through the sponge body and all the choanocytes line the central large space called the ‘spongocoel’. The water enters the ostia, is drawn through to the spongocoel and leaves through a single large osculum. Asconoid sponges have cylindrical hollow bodies and tend to grow in groups attached to some object or other in relatively shallow seas.

Most modern sponge species are Leuconoid. In leuconoid sponges, the canal system is more complicated again, with the canals being longer and more branched, they lead to special chambers whose walls are lined by choanocytes; there are no choanocytes in the canals. There is no real spongocoel just a central exit canal leading to the osculum. Leuconoid sponges tend to live in large groups with each individual sponge having its own osculum, however the borders between individual sponges are often hard to define and the sponge may act more like a large communal organism.

1. Coelenterates (Cnidaria):
   

Cnidaria is a phylum containing some 11,000 species of relatively simple invertebrate
animals found exclusively in aquatic, mostly marine, environments. Cniderians include sea anemones, corals, jellyfish, sea pens, sea pansies, sea wasps, and tiny freshwater hydra.

Here are the major characteristics of Cnidaria:

• Radially Symmetrical;
  
• Body multicellular, few tissues, some organelles;
  
• Body contains an internal cavity and a mouth;
  
• Two different forms exist, medusa and polyp;
  
• Reproduction is asexual or sexual;
  
• Has a simple net like nervous system;
  
• Has a distinct larval stage which is planktonic;
  
• Lives in aquatic environments, mostly marine;
  
• Mostly carnivorous otherwise filter feeders; and
  
• May have a minimal skeleton of chiton or calcium carbonate.
  

The basic body shape of a cnidarian consists of a sac with a gastro vascular cavity, with a single opening that functions as both mouth and anus. It has radial symmetry, meaning that whichever way it is cut along its central axis (that is, by any plane that passes through its longitudinal axis); the resulting halves would always be mirror images of each other. Organisms with bilateral symmetry, such as humans, can only be divided into similar halves by one specific plane passing through the longitudinal axis.

The movement of a cnidarian is coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess rhopalia, complex sensory structures that can include image-forming eyes with lenses and retinas, and a gravity-sensing statolith comparable in function to the otolith of the vertebrate inner ear. Tentacles surrounding the mouth contain nematocysts, specialized stinging cells, which they use to catch prey and defend themselves from predators. The ability to sting is what gives cnidarians their name.

There are four classes of Cnidaria:

2.1    Class Anthozoa:

This class of Anthozoa (Figure: 04) includes Anemones, Sea Fans, Corals, etc.

Anthozoa is a class of marine invertebrates within the phylum Cnidaria that are unique among cnidarians in that they do not have a medusa stage in their development. These exclusively polypoid cnidarians are characterized by a tubular body with tentacles around the mouth and most are sedentary after the larval stage. Anthozoa includes the sea anemones, corals, sea pens, sea pansies, and sea fans, among others.

Anthozoa is the largest of the four classes of Cnidaria with over 6,000 species. They are found worldwide in all oceans, from the Arctic to the Antarctic. Anthozoa means, “Flower animals,” which is descriptive of this class of invertebrates. Anthozoans provide a number of values for human beings. Coral reefs are major tourist attractions and provide a habitat for fish, mollusks, urchins, and crustaceans that serve as food for people. Anthozoans are used in the aquarium trade, to make coral jewellery, and scleractinian skeletons are even used as building materials and in bone grafts. Despite these values, various human activities (fishing, development, marine pollution) have had negative effects on coral reefs, with more than half of the world’s coral reefs considered to be threatened;

2.2    Class Hydrozoa:

Hydrozoans include marine hydroids, freshwater hydras, some known as jellyfish and corals, and the well-known Portuguese man-of-war (or Portuguese man o’ war).

Hydrozoa (figure: 05) is a diverse and wide ranging taxonomic class (sometimes super class) of marine and freshwater
invertebrates within the phylum Cnidaria, whose members are characterized by a life cycle that always includes the presence of planula larva, and the medusa, if present, having a velum, or muscular projection from the subumbrellar margin. Hydrozoans generally display alternation of generations between polyp and medusa, although hydras exhibit only the polyp form and some species are represented only by medusae and lack the polyp stage. Hydrozoans are carnivorous animals that can be solitary or colonial. Most are small (an umbrella of less than 50 millimetres or two inches), but some can be large (40 centimetres or 17.7 inches), and some colonies can be very large (30 meters or 98 feet).

While the often small and diaphanous hydrozoa, as polpys or medusae, often go unnoticed, they are important in aquatic food chains. Hydrozoans capture crustaceans, among other appropriately sized animals, with the medusae sometimes feeding extensively on fish eggs and larva, and these invertebrates are preyed upon by various fish, crustaceans, and mollusks. While the characteristic cnidarian stinging structures known as nematocysts provide protection from many predators, some sea slugs are able to appropriate the nematocysts for their own defence;

2.3    Class Scyphozoa (True Jellyfish):

Jellyfish (Figure: 06) are marine invertebrates belonging to the Scyphozoan class of the Cnidaria phylum. The body of an adult jellyfish is composed of a bell-shaped, jellylike substance enclosing its internal structure, from which the creature’s tentacles suspend.

Since jellyfish do not biologically qualify as actual “fish“, the term “jellyfish” is considered a misnomer by some, who instead employ the names “jellies” or “sea jellies”. The name “jellyfish” is also often used to denote two other classes of cnidarians, Class Hydrozoa (Portuguese Man o’ War, Obelia, etc.), or Class Cubozoa (box jellyfish or box jellies). Jellyfish can be found in every ocean in the world. The size of the cups or bells can range from two centimetres across to four meters, and some species trail tentacles for many meters below in the water. Most jellyfish are passive drifters.

Theoretically, members of Cnidaria have life cycles that alternate between asexual polyps (the body as a vase shaped form), and sexual, free-swimming forms called medusae (singular medusa; the body in a bell-shaped form). However, the Scyphozoa live most of their life cycle as medusa. The class name Scyphozoa means “cup animals,” reflective of the dominant medusa form. The term comes from the Greek word skyphos, denoting a kind of drinking cup.

There is some concern that blooms of jellyfish—congregating of hundreds and even thousands of these animals—correlates with such anthropogenic impacts as over-fishing and pollution; and

2.4    Class Cubozoa (Box Jellies):

Box jellyfish (Figure: 07) is the common name for any of the radially symmetrical, marine invertebrates comprising the Cnidarian
class Cubozoa, characterized by generally well-developed eyes and a life cycle dominated by a cube-shaped medusa stage. The well-known cubozoan species, Chironex fleckeri, which is sometimes called the box jellyfish, is among the most venomous creatures in the world. The name sea wasp is also applied to Chironex fleckeri and some of the other cubozoan species. Members of Cubozoa, collectively, are known scientifically as cubazoans and commonly as box jellies.

With bodies shaped roughly like a square bell with tentacles dangling from the corners, box jellies are agile and active swimmers based on their ability to contract the “bell” and forcefully expel water through a constricted opening at its base. Box jellies are important components of marine ecosystems, capturing and eating fish, crustaceans, and worms, and despite their barbed and poisoned nematocysts, being eaten by large fish and sea turtles.

Box jellies frequent coastal tropical waters, especially around Vietnam, Hawaii, Australia, and the Philippines. Between 1954 and 1996, more than 5000 human lives were lost due to their venomous sting. Nonetheless, while the box jellies can be highly dangerous to swimmers, divers, and surfers who enter their habitat, they also add to the wonder and mystery of nature for people with their unique form and behaviour.

1. Echinoderms (Echinodermata):
   

The phylum Echinodermata, which contains about 6000 species, gets its name from the Greek, literally meaning “spiny skin.” This phylum exists exclusively in the sea and includes sea stars, urchins, brittle stars, and sea cucumbers. They are simple animals, lacking a brain and complex sensing organs. Echinoderms are characterized by their radial symmetry, water vascular system and internal skeleton.

Echinoderms form a well-defined and highly derived clade of metazoans. They have attracted much attention due to their extensive fossil record, ecological importance in the marine realm, intriguing adult morphology, unusual biomechanical properties, and experimentally manipulable embryos. The approximately 7,000 species of extant echinoderms fall into five well-defined clades: Crinoidea, Ophiuroidea (basket stars and brittle stars), Asteroidea (starfishes), Echinoidea, and Holothuroidea. The phylogenetic position of the Concentricycloidea remains controversial.

Here are the major characteristics of Echinodermata:

• Possess 5-rayed symmetry, mostly radial, sometimes bilateral;
  
• Body has more than two cell layers, tissues and organs;
  
• Body cavity a true coelom;
  
• Most possesses a through gut with an anus;
  
• Body shape highly variable, but with no head;
  
• Nervous system includes a circum-oesophageal ring;
  
• Has a poorly defined open circulatory system;
  
• Possesses a water vascular system, which hydraulically operates the tube feet or feeding tentacles;
  
• Without excretory organs;
  
• Normally possesses a sub-epidermal system of calcareous plates;
  
• Reproduction normally sexual and gonochoristic;
  
• Feeds on fine particles in the water, detritus or other animals; and
  
• All live marine environments.
  

Most species reproduce sexually, and species have separate sexes. Fertilization is external; the gametes are simply shed into the water at spawning time. The floating embryo develops into a ciliated, free-swimming, bilaterally symmetrical larva, which undergoes metamorphosis into the radially symmetrical adult.

Here are six classes of echinoderms:

3.1    Class Asteroidea:
Sea stars, or starfish, vary in shape from nearly circular to pentagonal, to the familiar starlike, and flowerlike forms with five or more tapering arms. The arms are extensions of the body; each contains an extension of the body cavity, a radial canal, and body organs. Each arm has an ambulacral groove on the undersurface; in the furrow of the groove is the ambulacral area, or ambulacrum, with holes for the tube feet. The margins of the groove have spines that can close over the ambulacrum. The tip of each arm bears a tube foot that functions as a sensory receptor for chemical and vibratory stimuli, and some have a red pigment spot that serves as a simple eye. The outer surface consists of a latticework of lime ossicles, or plates, between which projects thin-walled fingerlike extensions called papulae.

The papulae and the tube feet are the principal sites of respiratory exchange. In some groups of sea stars there are also body wall projections called pedicellaria, equipped with tiny pinchers that are operated by muscles and are used to clean the body surface and capture very small prey. Sea stars crawl about on rocks or muddy bottoms, feeding on a variety of living and dead animals. Many feed largely on bivalve mollusks and are notorious as destroyers of commercial oyster beds. There are two or more gonads in each arm; at spawning time, these may nearly fill the arms. The swimming larva settles and goes through a sessile (attached) stage while changing to the adult form.

Here is an example of Asteroidea (Figure: 08):

3.2    Class Ophiuroidea:
The brittlestars, or serpent stars, are so called for their long, slender, fragile arms, which are set off sharply from the circular, pentagonal, or slightly star-shaped body disk. The arms of brittlestars are flexible and appear jointed because of the conspicuous plates of the outer surface (Figure: 09).

They bear a row of spines along each edge. In one group, the basket stars, they are repeatedly branched, forming a large mass of tentaclelike limbs. Each arm contains a radial canal (or one of its branches), but it does not contain body organs. Brittlestars feed on detritus and small organisms. The mouth leads to a large saclike stomach that fills most of the body cavity. There is no intestine or anus, and solid waste is extruded through the mouth. The stomach is folded into ten pouches, between which lie ten respiratory sacs that open by slits onto the oral surface. The cells lining the sacs have flagellae, which create a current of water moving in and out. Respiratory exchange occurs chiefly through the thin lining of the sacs;

3.3    Class Echinoidea:
Echinoids, heart urchins, and sand dollars —are echinoderms without arms and with a spiny shell, or test, formed of tightly fused skeletal plates. The sea urchins (regular echinoids) are hemispherical in shape, round on top and flat on the lower surface. They have very long, prominent spines and are often brightly coloured. The test of a sea urchin is divided into ten parts from pole to pole, like the sections of an orange. Five of these are ambulacra, with openings for tube feet; these alternate with wider sections, called interambulacra that lack tube feet (Figure: 10).

However, spines and pedicellaria are found over the entire surface of the test. Urchins move by pushing against the substratum with the spines and extending the tube feet in the direction of movement. If turned over they can right themselves by means of the tube feet on the aboral surface. The mouth, located in the center of the undersurface, is surrounded by a thickened region bearing five pairs of short, heavy tube feet and sometimes five pairs of bushy gills. Within the mouth is an elaborate five-sided jaw structure called Aristotle‘s lantern that can be partially extruded from the mouth. It is able to grind up calcareous exoskeletons of plants and animals. The anus is at the center of the aboral surface and is surrounded by a thin-walled area without skeletal plates.

Sand dollars and heart urchins (irregular echinoids) have a dense covering of short spines, and locomotion is exclusively by movement of the spines. There are two groups of podia-bearing ambulacra, one arranged in a petallike pattern on the upper surface and the other forming a similar pattern on the lower surface. The upper tube feet function as respiratory organs (there are no gills around the mouth), and the lower ones are specialized for gathering food particles. Sand dollars are extremely flattened and oval in outline; the anus is on the oral surface. Heart urchins are somewhat flattened and are heart-shaped; a deep ambulacral groove running from top to bottom creates a secondary bilateral symmetry. The anus is on the aboral surface, opposite the groove;

3.4    Class Holothuroidea:
The sea cucumbers are long-bodied echinoderms with the mouth at or near one end and the anus at or near the other. Because of their elongation along the oral-aboral plane, they lie on their sides rather than on the oral surface.

In nearly all sea cucumbers, the skeleton is reduced to microscopic ossicles imbedded in the leathery skin. Sea cucumbers have no arms, but tube feet around the mouth have been modified to form a circle of 10 to 30 tentacles of varying lengths and shapes that function in gathering food particles from the ocean bottom. The gut of the sea cucumber terminates in a chamber called the cloaca that opens into the anus. Two unique structures called respiratory trees, found in most sea cucumbers, also terminate in the cloaca.

These are systems of highly branched tubes, one on either side of the body. The animal pumps water into the respiratory trees by contracting the cloaca, and oxygen diffuses through from the walls of the trees into the fluid of the body cavity. The madreporite in most sea cucumbers opens into the body cavity rather than to the outside and receives its fluid from the cavity. In a few sea cucumber species there is a large mass of tubules at the base of the respiratory tree that can be shot out of the anus if the animal is irritated. The extruded tubules, which may engulf and incapacitate an intruder, break off; they are then regenerated by the sea cucumber. In other species the respiratory trees, gonads, and part of the digestive tract are shot out through the anus; this evisceration is followed by regeneration of the lost organs (Figure: 11);

3.5    Class Crinoidea:
The sea lilies and feather stars are members of an ancient group of stalked, sessile, detritus-feeding echinoderms. Most of the sea lilies remain stalked throughout life; their movements include bending the stalk and the arms and crawling. Feather stars break off the stalk and become free-living as adults. Crinoids, whether free or stalked, always have the oral side upward, and the ring of arms about the mouth gives them a flowerlike appearance. They have at least 10 arms, but some sea lilies have up to 40 and some feather stars up to 200 arms. The stalk and the arms have a jointed appearance, and each arm has a row of projections, the pinnules, on either side, giving a feathery appearance.

Here is an example of Crinoidea (Figure: 12):

A ciliated ambulacral groove runs along each arm and branches into the pinnules; the groove contains feathery, tube feet arranged in triads. These react to the presence of minute food particles in the water by bending inward, sweeping the particles into the groove, where they are trapped in mucus and swept by the cilia toward the mouth. Gametes develop in some of the pinnules, which rupture at spawning time. The free-swimming larva eventually settles and develops a stalk and a crown; and

3.6    Class Concentricycloidea:
The sea daisies, which were discovered in 1986, have disk-shaped flat bodies and are less than 0.39 in. (1 cm) in diameter. The two known species were located on wood found in deep waters off the coasts of New Zealand and the Bahamas (Figure: 13). They have a water-vascular system, with tube feet on the body surface around the edge of the disk. They have no obvious arms or mouth, and appear to absorb nutrients through the membrane surrounded their bodies.

1. Flatworms (Platyhelminthes):
   

Platyhelminthes, phylum containing about 20,000 species of soft-bodied, bilaterally symmetrical, invertebrate animals, commonly called flatworms. While the structure of the flatworms marks a major step in animal evolution, their origin and relationships within the group are still controversial.

Flatworms are dorso-ventrally flattened. The epidermis is generally ciliated in the turbellarians, while trematodes and cestodes are covered with a cuticle. Beneath the outer covering are two layers of muscle, an outer circular layer, and an inner longitudinal layer; this arrangement permits an undulating form of locomotion that can be observed in the larger turbellarian species. A saclike digestive cavity, with a single opening to the outside that serves as both mouth and anus, is sometimes present; in the simpler forms, it is absent or unbranched, but in higher forms, it branches to all parts of the body. The major sense organs, when present, are concentrated in the head, or front end. Although a primitive nerve net is present in some of the simpler forms, others have several nerve cords extending from a brain along the length of the body. The latter pattern of organization is retained in the nervous systems of higher invertebrates, specifically annelids and arthropods.

Here are the major characteristics of Platyhelminthes:

• Bilaterally symmetrical;
  
• Body having 3 layers of tissues with organs and organelles;
  
• Body contains no internal cavity;
  
• Possesses a blind gut (i.e. it has a mouth but no anus);
  
• Has Protonephridial excretory organs instead of an anus;
  
• Has normally a nervous system of longitudinal fibres rather than a net;
  
• Generally dorsoventrally flattened;
  
• Reproduction mostly sexual as hermaphrodites;
  
• Mostly they feed on animals and other smaller life forms; and
  
• Some species occur in all major habitats, including many as parasites of other animals.
  

There are four classes: the free-living, primarily aquatic class, Turbellaria, and Trematoda, Cestoda, and Monogenea, which are considerably modified for their exclusively parasitic existence.

4.1    Class Turbellaria:

Here is an example of Turbellaria (Figure: 14):

A soft epidermis that is ciliated, at least on the ventral surface characterizes the mostly free-living, primarily carnivorous, flatworms of class Turbellaria. The movement of the cilia propels the smaller forms. Larger species glide along by muscular waves, usually over mucous beds secreted by special cells.

Turbellarians are generally divided into five groups based primarily on differences in the form of the digestive cavity, a structure that is readily observable through the transparent body wall. The most primitive turbellarians, the acoels, have no digestive cavity. The ventral mouth, and sometimes a simple pharynx lead to an inner mass of nutritive cells. Most species measure less than 1/8 in. (3 mm) in length.

The rhabdocoels have straight, unbranched digestive cavities. Some authorities believe that the rhabdocoels gave rise to both the trematodes and cestodes because several rhabdocoels species exhibit commensal relationships, which presumably could have given rise to parasitism. The allocoels were formerly classified together with the rhabdocoels; the gut can be either saclike or branched.

The triclads, also known as planarians, are relatively large flatworms named for their three-branched gut. Most species range from 1/8 in. (3 mm) to about 1 in. (2.5 cm) in length. Planarians have more sense organs and a more complex brain than the other turbellarians. The freshwater species Dugesia tigrina has primitive eyes and tactile lobes, or auricles, on the sides of the head. The muscular pharynx can be extruded for food capture. Dugesia and many other planarians can regenerate entirely new individuals from small pieces cut from the body;

4.2    Class Trematoda:
The parasitic flatworms of class Trematoda, also called flukes, have oral suckers, sometimes supplemented by hooks, with which they attach to their vertebrate hosts. Trematodes (Figure: 15) have retained the same body form and digestive cavity as the turbellarians. However, practically the entire interior is occupied by the reproductive system; the organism is capable of producing huge numbers of offspring. Trematodes of the order Digenea have complex life cycles involving two or more hosts. The larval worms occupy small animals, typically snails and fish, and the adult worms are internal parasites of vertebrates. Many species, such as the liver fluke Clonorchis sinensis and the blood fluke (Schistosoma), cause serious diseases in humans;

4.3    Class Cestoda:
The body of the cestodes (Figure: 16), also known as tapeworms, has lost the typical turbellarian form. Although there are a few unsegmented species, the bulk of a typical cestode body consists of a series of linearly arranged reproductive segments called proglottids. There is no mouth or digestive system; food is absorbed through the cuticle. Adults live in the digestive tract of vertebrates, and larval forms encyst in the flesh of various vertebrates and invertebrates.

The body of an adult tapeworm is virtually a reproductive factory. Behind a small securing knob, called a scolex, which bears a circle of hooks or other attachment organs, the proglottids constantly bud off and gradually enlarge. As they mature, they become filled with male and female reproductive organs. Cross-fertilization takes place with adjacent worms or neighbouring proglottids; in some cases, self-fertilization occurs. In some species, the ripe proglottids, filled with eggs, are shed.

In others, the fertilized eggs leave the adult host in the feces. If the intermediate host consumes the eggs, the life cycle continues. Tapeworm species that infest human intestines as adults include Taenia saginata, T. solium, the dwarf tapeworm, Hymenolepsis nana, and the fish tapeworm, Diphyllobothrium latum, which can reach lengths of up to 50 ft (15 m); and

4.4    Class Monogenea:
Here is an example of Monogenea (Figure: 17):

Monogenetic flukes spend their entire life cycle as parasites on a single host, often on the gills and skin of fish; they include no human parasites. They hold on to the fish by the use of hooks and attachment organs at the posterior end. Most of the parasite’s body space is devoted to the hermaphroditic reproductive system. The egg on hatching releases a ciliated larva that enables the parasite to reach a new host. Species of the genus Gyrodactylus can be serious pests in hatcheries, particularly since a single worm can give rise to more than one hundred descendants in three weeks.

5.    Roundworms (Nematoda):

Here are the major characteristics of Nematoda:

• Bilaterally symmetrical, and vermiform;
  
• Body has more than two cell layers, tissues and organs;
  
• Body cavity is a pseudocoel, body fluid under high pressure;
  
• Body possesses a through gut with a sub-terminal anus;
  
• Body covered in a complex cuticle;
  
• Has a nervous system with pharyngeal nerve ring;
  
• Has no circulatory system (no blood system);
  
• Reproduction normally sexual and gonochoristic;
  
• Feed on just about everything; and
  
• Live just about everywhere, many species are endoparasites.
  

The Phylum Nematoda consists of the species commonly known as roundworms. There are approximately 12,000 described species, but the actual number could be many times higher. Nematode worms are extremely abundant; often, several hundred species, and as many as a million individuals, inhabit a square yard of soil.

Nematodes are also extremely varied ecologically. They are found in almost every imaginable habitat, including terrestrial (land-based), freshwater, and saltwater ecosystems, as well as within other organisms as parasites. Nematodes can be herbivorous, carnivorous, or parasitic, and include both generalists (who make use of a wide variety of resources) and specialists (who make use of only particular resources). They play a particularly critical role in decomposition and nutrient cycling, where they are often the intermediate decomposers that partly break down organic materials so that they can then be dealt with by bacterial decomposers.

Some well-known nematode parasites include hookworms, pinworms, and heartworms. Also included are Trichinella spiralis, which is responsible for trichinosis and uses both pigs and humans as hosts, and filarial worms, which are the primarily tropical parasites responsible for the diseases elephantiasis and river blindness.

Roundworms are small, slender, unsegmented worms, which are tapered at both ends. They have a circular cross section. Different species of nematodes are often difficult to distinguish because of their uniform external morphology, or outer appearance.

Nematodes are characterized by an external (outer) layer of cuticle that is secreted by the hypodermis underneath it. The cuticle is somewhat rigid. However, it is flexible enough to permit bending and stretching, and can be penetrated by gases and water. The cuticle is molted, or shed, several times during the worm’s growth. The hypodermis underlying the cuticle is a syncitium—that is, it consists of large cells with more than one nucleus. A layer of muscle cells is found beneath the hypodermis.

All nematode muscle fibers run lengthwise along the animal’s body. This single, unvaried orientation limits nematodes to their characteristic, and somewhat awkward, pattern of movement, a flailing whip like motion that is produced by alternate contractions (shortenings and thickenings) of muscle cells on either side of the animal’s body. The rigidity of the cuticle layer also limits the motion of nematodes.

Nematodes lack a true coelom (body cavity) since their internal cavity is not lined by cells originating from the embryonic mesoderm. Instead, they possess a fluid-filled pseudocoel (incomplete coelum) that contains the intestine and reproductive organs. The nematode nervous system is characterized by a rear nerve ring around the area of the pharynx (area deep inside the mouth cavity) and two pairs of lengthwise nerve cords that run down the body. There are also dorsal (back) and ventral (belly) nerve cords as well as a set of lateral nerve cords across the body. These nerve cords transmit sensory information and coordinate movement. Nematodes have a variety of sensory receptors, including tactile (touch) receptors at the front and back ends of the body, and chemosensory (chemical-sensitive) cells at the front end. They also have light-sensitive organs organized either in ocelli (simple eyes) or distributed along the surface of the body.

Nematodes have a complete gut with a mouth and an anus. Teeth, which are used to pierce animal or plant matter, aid in obtaining food. The pharynx is muscular and pumps food through the gut, and nutrients are absorbed in the intestine. There is no internal system of circulation, so the transport of nutrients and wastes is achieved by diffusion (scattering). Specialized cells for excretion, which are known as renette cells and are unique to the phylum, remove nitrogen-laden wastes. These are expelled from the nematode directly through the body wall, in the form of ammonia.

Nematodes breathe across their entire body surface. This gas exchange strategy is adequate because of the small size of the worms, which means they have a high ratio of surface area to volume.

The majority of nematodes are dioecious; that is, the sexes are separate. Some species, however, are hermaphroditic, having both male and female reproductive organs. In dioecious species, males have a specialized spine for sexual reproduction that is used to open the female’s reproductive tract and to inject sperm. Nematode sperm is unusual in that the sperm cells do not have flagella, and move using an amoeboid motion (crawling). While some species are live bearing, most lay eggs. Eggs escape through a midbody hole called the gonopore in the female. There is no distinct larval stage. Eggs develop directly into juveniles that generally resemble the adults except that they lack mature reproductive organs. An unusual feature called “eutely,” in which every individual of a given species has exactly the same number of cells, also characterizes nematodes. This cell number is achieved by the end of the developmental period, so that subsequent growth of the animal involves increases in cell size rather than in cell number.

1. Segmented Worms (Annelida):
   

The annelids, collectively called Annelida and a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments. Annelids are considered members of the Lophot rochozoa, a “super-phylum” of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans.

Here are the major characteristics of Annelida:

• Bilaterally symmetrical and vermiform;
  
• Body has more than two cell layers, tissues and organs;
  
• Body cavity is a true coelom, often divided by internal septa;
  
• Body possesses a through gut with mouth and anus;
  
• Body possesses 3 separate sections, a prosomium, a trunk and a pygidium;
  
• Has a nervous system with an anterior nerve ring, ganglia and a ventral nerve chord;
  
• Has a true closed circulatory system;
  
• Has no true respiratory organs;
  
• Reproduction normally sexual and gonochoristic or hermaphroditic;
  
• Feed a wide range of material; and
  
• Live in most environments.
  

No single feature distinguishes Annelids from other invertebrate phyla, but they have a distinctive combination of features. Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called annuli and inter nally by septa (“partitions”) at the same points, although in some species the septa are incomplete and in a few cases missing. Most of the segments contain the same sets of organs, although sharing a common gut, circulatory system and nervous system makes them inter-dependent. Their bodies are covered by a cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen and does not molt on the other hand arthropods’ cuticles are made of the more rigid a-chitin, and molt until the arthropods reach their full size. Most annelids have closed circulatory systems, where the blood makes its entire circuit via blood vessels.

Over 17,000 modern species are grouped into the following three classes:

6.1    Class Ragworms:

A ragworm (Figure: 22) is a type of annelid worm which lives along the Atlantic shoreline in both North America and Europe. The worms can also be found in parts of the Mediterranean, and they are extremely abundant, providing a source of food to many wading birds and larger ocean creatures. Some fishermen are familiar with ragworms, as they are commonly used as bait, and they are available at many fishing supply stores in both fresh and frozen states.

Depending on regional dialects, ragworms may also be known as sandworms or clamworms. Biologists classify a large number of worms in the Nereidae family as ragworms, especially those in the Nereis genus. There are a number of physical differences between the various species of ragworms, but they all share the segmented bodies, which are a crucial classifying characteristic for annelid worms. Ragworms also tend to be slightly flattened, and they are extremely good at burrowing, making J or S shaped holes in the sand for shelter.

Depending on the species, a ragworm may be an active predator, pursuing an assortment of small ocean creatures and sifting through the mud for food, or it may be a scavenger. Some ragworms mix both behaviours, opportunistically taking advantage of whatever food comes their way. Ragworms also line their bodies with stiff bristles, which the animals use to navigate in the ocean and mud that they call home.

These worms can live from one to three years, depending on how long it takes them to reach maturity. Once ragworms reach their maturity, they spawn once before dying, typically producing a large number of fertile eggs which will develop into a new generation of ragworms. Like many worms, the ragworm has evolved over millions of years, establishing a system of life and reproduction, which seems to be quite effective for these abundant animals;

6.2    Class Earthworms:

The earthworm (Figure: 23) family includes several different species of invertebrates that live in most parts of the world. These animals can vary significantly in terms of size, with some being as small as half an inch (about a centimeter) and others being over 20-feet-long (about 6 meters). They are generally brownish in color, and they have segmented bodies, which contract and relax to produce simple locomotion. Animals in the earthworm family are known to eat nutrients left over by decomposing plant matter in soil, and most scientists think they are very important to the ecosystem. Earthworms are generally blind and deaf, but their skin is covered in cells that allow them to taste the soil and detect light.

All earthworms generally have reproductive parts from both sexes. This means that they can produce both sperm and eggs, and when they mate, both worms produce separate offspring. The earthworm mating process generally involves the worms tangling up with each other and excreting mucus that allows each worm to absorb the other’s sperm. After that, the worms generally use the sperm to fertilize eggs inside their bodies. These eggs are released inside mucus rings that form on the outside of each worm’s body and eventually fall to the ground, where they become protective casings.

All earthworms need to survive is moist soil and warm temperatures. They are coldblooded, so they cannot survive in areas where the temperatures are too severe. They also generally need a certain amount of moisture, and if they get too dry, they will die. Animals in the earthworm family also have quite a few natural enemies. For example, some bird species eat earthworms as their main food source, and these birds often have special adoptions in their vision that allow them to detect the movements of earthworms beneath the surface; and

6.3    Class Leeches:

Leeches are carnivorous invertebrates in the class Hirudinea. A few specific leeches feed on blood, which has made this large and rather diverse group of creatures famous with humans. Bloodsucking leeches have been used in medical treatment for thousands of years, and they continue to be used in certain circumstances today. Humans also encounter leeches in the wild, where they are irritating but not usually harmful.

These animals are annelids, which means that their bodies are divided into segments. When a leech is viewed under a microscope, the segments can be clearly seen. Leeches are also hermaphroditic, like many invertebrates. They tend to be aquatic, living in freshwater, swamps, and marshes. Some leeches are also comfortable on land, especially in humid areas like jungles.

Many leeches are carnivorous, feeding on smaller invertebrates. Others scavenge for various organic materials, while some leeches attach themselves parasitically to other animals to feed on their blood. Leeches will feed on fish, reptiles, waterfowl, amphibians, and mammals, depending on available sources of potential food in their areas. Bloodsucking leeches secrete special chemicals, which open blood vessels, inhibit clotting, and numb the wound so that their hosts are not aware of their presence until they are already gone.

Historically, bloodletting was an important aspect of medical practice, prescribed for a wide range of conditions. One form of bloodletting involved the use of leeches, and “leech” was actually a common slang term for “doctor” at one point. In modern medicine, bloodletting for therapeutic purposes is varying rare, but the use of leeches is actually not uncommon. They may be used at surgical sites to promote circulation and the flow of blood, for example, and they are used in the treatment of frostbite and other circulatory conditions.

7.    Mollusks (Mollusca):

Mollusca are one of the most diverse groups of animals on the planet, with at least 50,000 living species (and more likely around 200,000). It includes such familiar organisms as snails, octopuses, squid, clams, scallops, oysters, and chitons. Mollusca also includes some lesser known groups like the monoplacophorans, a group once thought to be extinct for millions of years until one was found in 1952 in the deep ocean off the coast of Costa Rica. Molluscs are a clade of organisms that all have soft bodies that typically have a “head” and a “foot” region. Often their bodies are covered by a hard exoskeleton, as in the shells of snails and clams or the plates of chitons.

Here are the major characteristics of Mollusca:

• Bilaterally symmetrical;
  
• Body has more than two cell layers, tissues and organs;
  
• Body without cavity;
  
• Body possesses a through gut with mouth and anus;
  
• Body monomeric and highly variable in form, may possess a dorsal or lateral shells of protein and calcareous spicules;
  
• Has a nervous system with a circum-oesophagal ring, ganglia and paired nerve chords;
  
• Has an open circulatory system with a heart and an aorta;
  
• Has gaseous exchange organs called ctenidial gills;
  
• Has a pair of kidneys;
  
• Reproduction normally sexual and gonochoristic;
  
• Feed a wide range of material; and
  
• Live in most environments.
  

A part of almost every ecosystem in the world, molluscs is extremely important members of many ecological communities. They range in distribution from terrestrial mountain tops to the hot vents and cold seeps of the deep sea, and range in size from 20-meter-long giant squid to microscopic aplacophorans, a millimeter or less in length, that live between sand grains.

These creatures have been important to humans throughout history as a source of food, jewellery, tools, and even pets. For example, on the Pacific coast of California, Native Americans consumed large quantities of abalone and especially owl limpets. However, the impact of Native Americans on these molluscan communities pales by comparison to the over harvesting of some molluscan taxa by the United States in the 1960s and 1970s. Species, whose members once numbered in the millions, now teeter on the verge of extinction.

For example, fewer than 100 white abalone remain after several million individuals were captured and sold as meat in the 1970s. Besides having yummy soft parts, molluscs often have desirable hard parts. The shells of some molluscs are considered quite beautiful and valuable. Molluscs can also be nuisances, such as the common garden snail; and molluscs make up a major component of fouling communities both on docks and on the hulls of ships. They also have a very long and rich fossil record going back more than 550 million years, making them one of the most common types of organism used by paleontologists to study the history of life.

New types of data and much larger and more sophisticated analyses continue to be performed. The resolved relationships shown (such as cephalopods, scaphopods, and gastropods) are recent discoveries:

7.1    Class Bivalvia (Scallops, Clams, Mussels, etc.):

Anyone that has ever slurped down an oyster, worn a pearl around their neck, or enjoyed the half-shells found on the beach has encountered bivalves. However, bivalves are much more than that. The second most diverse group of molluscs behind gastropods, bivalves are one of the most important members of most marine and freshwater ecosystems. In fact, there are well over 10,000 described species of bivalve, found from the deepest depths of the oceans, to the streams in your backyard.

Bivalves (Figure: 25) are easily recognized by their two-halved shell. They can burrow into the sediment or live on the ocean floor. Some can even move around through the water by snapping their shell open and shut to swim. Not all bivalves still have a shell though; some have evolved a reduced shell or have completely lost the shell.

Throughout history, bivalves have been one of the most important marine animals to humans. They have been used for food, jewellery, decoration, even money;

7.2    Class Monoplacophora (Limpet-like “Living Fossils”):

Here is an example of Monoplacophpra (Figure: 26):

Once known only from Paleozoic fossils, living monoplacophorans were discovered in 1952 in one of the most important discoveries in modern biology. Since that first recent species (Neopilina galathaea) was discovered, around 20 other species have been identified.

One reason for having evaded detection for so long is that they are generally found in the deep ocean. Finding them has been quite a boon to malacologists however, as monoplacophorans are often thought to be among the most primitive of molluscs. Indeed, many researchers believe that monoplacophoran-like ancestors gave rise to the rest of Mollusca.

Modern systematic research has borne out the idea of Monoplacophora being the basal member of the Mollusca clade. Their morphology then proves to be remarkably important in understanding what the first molluscs may have looked like, as well as how the other major groups such as bivalves and gastropods may have evolved. As only a few species of living monoplacophorans are known, and all being somewhat similar, much of our knowledge of the group comes from fossils;

7.3    Class Gastropoda (Snails, Slugs, Limpets, Sea Hares):

Gastropods are one of the most diverse groups of animals, both in form, habit, and habitat. They are by far the largest group of molluscs, with more than 62,000 described living species, and they comprise about 80% of living molluscs. Estimates of total extant species range from 40,000 to over 100,000, but there may be as many as 150,000 species! There are about 13,000 named genera for both Recent and fossil gastropods. They have a long and rich fossil record from the Early Cambrian that shows periodic extinctions of subclades, followed by diversification of new groups.

Gastropods have figured prominently in pale-biological and biological studies, and have served as study organisms in numerous evolutionary, biomechanical, ecological, physiological, and behavioural investigations.

They are extremely diverse in size, body and shell morphology, and habits and occupy the widest range of ecological niches of all molluscs, being the only group to have invaded the land.

7.3.1    Class Cephalopoda (Squids, Octopuses, Nautilus, Ammonites):

Cephalopods (Figure: 28) are the most intelligent, most mobile, and the largest of all molluscs. Squid, octopuses, cuttlefish, the chambered nautilus, and their relatives display remarkable diversity in size and lifestyle with adaptations for predation, locomotion, disguise, and communication. These “brainy” invertebrates have evolved suckered tentacles, camera-like eyes, color-changing skin, and complex learning behaviour.

Their lengthy evolutionary history spans an impressive 500 million years and the abundant fossils they have left behind (mostly shelled nautiloids and ammonoids) record repeated speciation and extinction events. From myths about their enigmatic fossilized remains to fantastic accounts of tentacled sea monsters, cephalopods also figure prominently in the literature and folklore of human societies around the world. Today, biologists and paleontologists continue to captivate the human mind and imagination with details of these molluscs’ behaviour, natural history, and evolution; and

7.3.2    Class Scaphopoda (Tusk Shells):

The Scaphopoda (Figure: 29) are a distinctive group of molluscs commonly known as the “tusk shells” because their shells are conical and slightly curved to the dorsal side, making the shells look like tiny tusks (see the photos below). The scientific name Scaphopoda means “shovel foot,” a term that refers to the “head” of the animal that lacks eyes and is used for burrowing in marine mud and sediments. The most distinctive feature of scaphopods is that the tubular shell is open at both ends, not just one end as in most molluscs.

Scaphopods live their adult lives buried in sand or mud, with their head end pointed downwards. Only the narrow posterior end of the shell sticks up into the seawater for water exchange and waste expulsion. Gills have been lost in the scaphopods, so the mantle tissue not only produces the shell, but also serves the function of gills in obtaining oxygen from seawater. The mantle is fused into a tube that surrounds the body of the animal, but it is open at both ends. Water is circulated around the mantle cavity by the action of numerous cilia. When the dissolved oxygen runs low, the water is ejected through the top end of the shell by contraction of the foot;

7.4    Class Aplacophora (Spicule-Covered, Warm-like Animals):

Aplacophora (Figure: 30) is a relatively small group of molluscs that are characterized by their lack of shells and worm-like appearance. Instead of a shell, their mantle secretes tiny calcareous spicules, which give them a beautiful shine. There are only about 320 species of these exclusively marine molluscs. There is a good chance that Aplacophora is a paraphyletic group. The two groups of aplacophorans are the footless Caudofoveata (also known as Chaetodermomorpha) and the Solenogastres (or Neomeniomorpha).

Unfortunately, this group is understudied and thus species-level diversity is severely underestimated in this poorly collected group. The knowledge base that does exist is remarkable when one considers that this is primarily a deep-water taxon. However, molecular phylogenetic studies of this taxon are still few and its placement on the molluscan tree is problematic; and

7.5    Class Polyplacophora (Chitons):

Chitons (Figure: 31) are the familiar group of organisms that have eight valves on their shells. A visit to any rocky intertidal habitat around the world will introduce one to these beautiful molluscs.

All chitons are marine and the group has a worldwide distribution. Most live in the rocky intertidal zone or shallow sub littoral (just below the low tide level), but some live in deep water to more than 7000 m. A few species are associated with algae and marine plants, and in the deep sea, waterlogged wood is a common habitat for one group.

Chitons are generally dioecious (have separate males and females), with sperm released by males into the water. In most chitons, fertilized eggs are shed singly or in gelatinous strings, and once fertilized in the water column, these develop into a trochophore larva (free-swimming and ciliated) that soon elongates and then directly develops into a juvenile chiton; there is no veliger stage (having a velum, a lobed, ciliate swimming organ). In brooding species, the eggs remain in the pallial cavity of the female where they are fertilized by sperm moving through with the respiratory currents. Upon hatching from the brooded eggs, the offspring may remain in the pallial cavity until they crawl away as young chitons or exit the pallial cavity as trochophores for a short pelagic phase before settling

Chitons are flattened, elongately-oval, with eight overlapping dorsal shell plates or valves, bordered by a thick girdle formed from the mantle that may be covered with spines, scales, or hairs. The pallial cavity, containing multiple pairs of small gills, surrounds the foot with which the animal typically clings to hard surfaces. The plates are greatly reduced or even internal in a few species, these species sometimes having an elongate, somewhat worm-like body. Most are small (0.5-5 cm, but 1 species reaches over 30 cm in length).

Chitons possess a heart and an open blood system, a pair of kidneys that open to the pallial cavity, a simple nervous system with two pairs of lateral nerve cords, and many special minute sensory organs (aesthetes) that pass through the shell valves. Some of these are specialized as light receptors, having a minute lens and retina-like structure. The mouth is surrounded by a simple velum and the head lacks tentacles or eyes. They feed on encrusting organisms such as sponges and bryozoans, and non-selectively on diatoms and algae that are scraped from the substrate with their radula, which is hardened by the incorporation of metallic ions. One group captures small crustaceans by trapping them under the anterior part of their body.

8.    Arthropods (Arthropoda):

The arthropods include crustaceans, insects, centipedes, millipedes, symphylans, pauropodans, and the extinct trilobites. Arthropods are characterized by a segmented body covered by a jointed external skeleton (exoskeleton), with paired jointed appendages on each segment. A complex nervous system with a dorsal brain, connective nerves passing around the anterior end of the digestive tract, and a ventral nerve cord with a ganglion in each body segment; an open circulatory system with a dorsal heart into which blood flows through paired openings (ostia); and a greatly reduced body cavity (coelom). Because the jointed exoskeleton blocks growth of the organism, it must be shed periodically. This phenomenon, called molting, or ecdysis, is a characteristic feature of the phylum; it permits rapid growth in size and significant change in body form until the new exoskeleton, secreted by the animal, has hardened. Arthropods are mainly terrestrial, but aquatic representatives are well known.

Here are the major characteristics of Arthropoda:

• Bilaterally symmetrical (in most cases);
  
• Body has more than two cell layers, tissues and organs;
  
• Body cavity a true coelom;
  
• Most possesses a through straight gut with an anus (in most cases);
  
• Body possesses 3 to 400+ pairs of jointed legs;
  
• Body possesses an external skeleton (in most cases);
  
• Body is divided in 2 or 3 sections;
  
• Nervous system includes a brain and ganglia;
  
• Possesses a respiratory system in the form of tracheae and spiracles (in most cases);
  
• Possesses a open or lacunnar circulatory system with a simple heart, one or more arteries, and no veins, (in most cases);
  
• Reproduction normally sexual and gonochoristic, but can be parthenogenetic;
  
• Feed on everything; and
  
• Live everywhere.
  

Here are six main classes:

8.1    Class Crustacea:

Members of class Crustacea (Figures: 32) are characterized by two pairs of antennae and two pairs of modified appendages (maxillae) used for food handling. There are over 40,000 species of crustaceans, including lobsters, shrimps, crayfish, crabs, copepods, barnacles, and a large number of minute planktonic forms. Crustaceans are the only arthropods that are mainly aquatic, and most of them are marine. Some have spread to humid areas near water. They use gills for respiration. The thoracic region typically bears walking legs (pereiopods), also used for capturing prey. The abdominal region often is equipped with swimmerets (pleopods) and a tail fan made up of a pair of appendages (uropods) and the telson. Their excretory organs are modified nephridia, as a rule producing dilute urine that contains a great deal of ammonia.

Crustaceans are herbivores, carnivores, or scavengers and are often vital elements of the food chain. Some, such as lobsters, shrimp, and crayfish, are important economically as edible shellfish. Barnacles are notorious as fouling organisms of ship bottoms and harbour installations. Some crustaceans are significant parasites of other aquatic organisms. As a rule, they pass through a complex set of molts during development, involving a series of larval stages. The characteristic larva is called a nauplius, with three pairs of appendages. More appendages are added as the organism passes through its developmental molts. The cuticle of crustaceans, unlike that of other arthropods, contains calcium deposits. The most familiar subclasses are the Branchiopoda–which includes the orders Notostraca (tadpole shrimps), Diplostraca (clam shrimps and water fleas), Ostracoda (ostracods), and Copepoda (copepods) and the Cirripedia (barnacles), a subclass–and the Malacostraca, which includes the orders Stomatopoda (mantis shrimps), Mysidacea (opossum shrimps), Isopoda (isopods), Amphipoda (amphipods), and Decapoda (crayfish, lobsters, shrimps, and crabs);

8.2    Class Chilopoda:

Class Chilopoda (Figure: 33) includes the 5,000 species of centipedes, all of which are terrestrial. Centipedes are carnivorous and predacious, immobilizing their prey, usually consisting of smaller arthropods, with the aid of their fangs. The body is composed of a head region bearing a pair of antennae, a pair of mandibles, and two pairs of maxillae, and a trunk region with one pair of legs on each segment. The anterior pair of trunk appendages (prehensors) is equipped with poison glands. Juveniles may have fewer appendages than adults may or may hatch with adult segmentation; new segments are added during developmental molts. Chilopoda are found throughout the globe in tropical as well as temperate climates;

8.3    Class Diplopoda:

There are about 8,000 species belonging to class Diplopoda (Figure: 34), which comprises the millipedes and is found worldwide. The head region has a pair of antennae, a pair of mandibles, and two pairs of maxillae that are usually fused into a single mouthpart, the chilognatharium. Millipedes possess a tracheal system for respiration. They are herbivores or scavengers on dead plant material. Glands that produce toxic or unpleasant compounds protect many;

8.4    Class Pauropoda:

There are about 500 known species belonging to class Pauropoda. Pauropods (Figure: 35) are soft-bodied, small (0.5—2.0 mm long), soil-inhabiting arthropods that are distributed worldwide. They are elongated and have 9—11 pairs of legs, but they have no trachea and no heart;

8.5    Class Symphyla:

Members of class Symphyla (Figure: 36) are rapid runners that range in length from 1 to 4 in. (2.5—10 cm). The class includes some 160 species. They are mainly scavengers on decayed vegetation, but one species, Scutigerella immaculata, is a serious pest of certain crops. Symphylans have twelve pairs of legs and resemble the centipedes;

8.6    Class Insecta:

Class Insecta is the largest of the arthropod classes, containing hundreds of thousands of species. Except for a few primitive or highly modified forms, insects are characterized by having one or two pairs of wings attached to the thorax. The head region bears a pair of antennae, a pair of mandibles, and two pairs of modified maxillae forming the mouthparts. The abdomen is well set off from the thorax and has no appendages except reduced ones that are modified as reproductive organs. The typical insect head bears compound eyes and one or more simple eyes and is covered by a continuous exoskeletal armor. The thorax is made up of three segments, each bearing a pair of legs. The last two segments usually bear a pair of wings. Insects are predominantly terrestrial and have tracheae for gas exchange. Unique excretory organs, known as Malpighian tubules, which are useful in conserving water, also characterize insects.

Members of the class are extremely varied. They have adapted too many different kinds of feeding and play a variety of important roles in their ecological communities. Mouthparts may be adapted to chewing either plant or animal food, for sucking plant sap or blood, or for lapping or swabbing moisture such as fruit juices or animal body fluids. Some burrow and feed in soil or plant tissue, some are runners or jumpers that feed at or near the ground level, and others feed on the wing.

Most primitive insects are wingless and have a relatively weak exoskeleton. These are forced to seek humid, protected habitats. Juveniles of primitive insects closely resemble the parents and undergo little change other than growth after hatching. This is called ametaboly. Many of the winged insects undergo paurometabolous development, hatching as nymphs that resemble the parent in many ways but that have small buds instead of wings. With each molt these juveniles change somewhat, and the wings increase in size as the young gradually assume the form of the adult. Some insects have adapted to an aquatic life to a certain extent, and in their juvenile stages, they are found in ponds and streams. Some of these are hemimetabolous; the juveniles are naiads, i.e., they resemble the nymphs of paurometabolous insects, but their wings do not grow during the juvenile molts, even though other body changes occur. Instead, the last molt before the adult stage is reached involves full development of the wings, after which the insect takes up a terrestrial existence.

The least primitive of the insects are termed holometabolous. In holometaboly, the eggs hatch to release the usually wormlike larvae, which are often equipped with false legs in the abdominal region to aid in locomotion. Wing buds are entirely lacking. Although the larvae grow at each molt, they do not begin to resemble the adult until later. During the larval stage, the young insect enters into a quiescent pupal stage. At the end of this stage, a major metamorphosis occurs, and the insect emerges with all the adult organs.

Insects often cause great losses in agriculture, attack stored products, parasitize humans and domesticated animals and plants, and serve as important carriers of disease organisms. They are also beneficial, producing honey and silk and pollinating the flowers of the majority of flowering plants.

Sources:

1. Encyclopaedia: http://education.yahoo.com/reference/encyclopedia/entry/Porifera
   
2. Tree of Life Web Project: http://tolweb.org/Porifera/2464
   
3. Earth Life: http://www.earthlife.net/inverts/porifera.html
   
4. New World Encyclopaedia: http://www.newworldencyclopedia.org/entry/Cnidaria
   
5. Encyclopedia: http://www.encyclopedia.com/topic/Echinodermata.aspx
   
6. Encyclopedia: http://www.encyclopedia.com/topic/Platyhelminthes.aspx
   
7. Book Pages: http://www.bookrags.com/research/nematoda-ansc-03/
   
8. ZipCodeZoo: http://zipcodezoo.com/Key/Animalia/Annelida_Phylum.asp
   
9. WiseGeek: http://www.wisegeek.com/what-is-ragworm.htm
   
10. University of California Museum of Palaeontology: http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php
    
11. Yahoo – Education: http://education.yahoo.com/reference/encyclopedia/entry/Arthropo
    
12. Yahoo – Education Arthropoda: http://education.yahoo.com/reference/encyclopedia/entry/Arthropo