(2) Scyphozoa coelenterates with an inconspicuous or absent polyp stage. The medusa may be very large. These are the typical jellyfishes.

(3) Actillozoa solitary and colonial forms with no medusae. The coelenteron contains vertical partitions (mesenteries). Corals have a calcareous sketeton secreted by the ectodrm and are frequently colonial. Sea anermones are usually solitary and never have a skeleton but they are nevertheless basically similar to corals and are included in the same class.

Coelom - Coelenterate animals like Hydra have a body wall with only two cell layers ectoderm and endoderm. Higher animals have another layer the mesoderm between these layers and. because they have three layers. they are called triploblastic animals. The mesoderm forms the muscles. reproductive organs, etc. In flatworms and other simple creatures it also forms a sort of packing tissue that separates the internal organs from the body wall andfrom each other. In more advanced animals, however, the mesoderm becomes split into two regions one surrounding the gut and other internal organs, the other lining the body wall. The fluid filled cavity between the two layers is the coelom. It is well developed in vertebrates, echinoderms, and many worms, but in arthropods and molluscs it is reduced and the main body cavity in these animals is really part of the blood system (a haemocoel). Animals with a coelom are said to be coelomate.

Coelomoduct - A duct of mesodermal origin connecting the coelom to the outside of the body e.g. the vertebrate oviduct. Eggs are shed into the coelom and then carried to the outside in the coelomoduct.

Coleoptera - Beetles. This order of Insecta is characterised by the possession of horny front wings (elytra) that meet in the mid line and normally cover all of the abdomen. All have biting jaws but they feed on a wide range of materials, some are fiercely carnivorous. Metamorphosis is complete, there being a larval stage very unlike the adult.

Collagen - A tough fibrous protein which binds various tissues together and also forms tendons and ligaments where high tensile strength is required.

Collembola - Order of primitive inects springtails.

Colon - Large intestine of vertebrates.

Comb Jellies - (See Ctenophora).

Commensalism - A state where two or more species of an animal live in fairly close association but without having much influence on each other. One species may benefit hut not both.

Commissure - Bundle of nerve fibres joining two parts of the central nervous system. E.g. circum oesophageal commissure joining the ganglia above and below the oesophagus of the earthworm.

Conditioned Reflex - The smell or taste of food makes a dog's saliva now. This is a normal reflex action involving no learning by the dog, but it can' be conditioned so that it occurs with a different stimulus. It a bell is sounded every time food is given the dog will associate the bell with food and soon the bell alone will cause
saliva to flow. This is the conditioned refex.

Condyle - Rounded projection of bone, especially on the skull, that fits into a socket on another bone to allow limited movement. The occipital condyles at the back of the skull articulate with the aflas vertebra and allow the head to be nodded.

Cone - Colour sensitive cell of the vertebrate retina.

Conjugation - A type of reproduction found in some protozoans such as Paramoecium. It involves paitial fusion of two individuals and the exchange of nuclear material Conjunctiva. Thin layer of tissue covering the front of the eye, and lining the eyelids of vertebrates.

Connective Tissue - This term is applied to various body tissues all of which bind cells and organs together. Connective tissue underlies the skin, surrounds nerves and muscles, joins bones and muscles to each other and often stores fat in its cells. Connective tissue cells are normally well spaced in a matrix of fluid in which there are numbers of collagen and other fibres. Tendons which join muscles to bones have to be very resistant to stretching and are made almost entirely of collagen, whereas other connective tissues may have very few collagen fibres.

Contractile Vacuole - Feature of many protozoans, especially fresh water species. There may be more than one in each organism and they gradually swell as they extract water from the surrounding protoplasm. Suddenly the vacuoles burst and force the water out of the body. They then begin to fill up again. This may be a means of discharging waste from the body but the main function appears to be the removal of water which continually passes into the body because of asmosis.

Conus Arteriosus - Muscular chamber of primitive vertebraie heart at beginning of ventral aorta.

Convergent Evolution - Hermann Melville, in his great sea story 'Moby Dick', wrote, To be short, then, a whale is a spouting fish with a horizontal tail. Whales do of course resemble fish. They are streamlined; they have a fluked tail, a pair of flippers as fore limbs, and they have warm blood and the females suckle their young with milk. Thus, despite all appearances, they are far more closely related to dogs, rabbits and Man himself than any fish.

One hundred and fifty million years ago another fish-like creature swam the seas. Though it became extirict 70 million years ago, fossils of its bones enable its, appearance to be reconstructed. But, far from being a fish, the preserved skeletons show beyon all doubt that the creature was a reptile. Because of its remarkable resemblance to a fish it was called ichthyosaur or fish lizard (Greek, ichthys, a fish; sauros, a lizard).The connecting link between whale, fish and ichthyosaur is; all have lived in the sea and during their evolution became adapted to an aquatic life. This evolutionary development, which often leads to apparent likenesses between stocks that have diverged a long way from an original ancestor, is called convergent evolution or simply convergence.

Convergence is a common phenomenon found in organisms living on land, in water and in the air. It provides powerful support for the theory of evolution by natural selection under the same selective pressures different groups will respond with apparently, similar adaptations. Care must be taken in classifying animals and plants: superficial resemblances between two organisms do not necessarily mean that they are closely related.

Cut off in the continent of Australia, the marsupials or pouched mammals have flourished for 150 million years. In most other parts of the world they have disappeared, for competition from the placental mammals proved too much.

Australia offers a wealth of environments and many different marsupials evolved, filling the niches.The similarity in appearance between the various marsupials and placental mammals elsewhere provides striking examples of convergence.

Thus Australia has its wolf the Tasmanina wolf the Tasmanian wolf, it has its native 'cat' (Dasyurus) and its native 'mouse' (Dasycercus). There are 'ant eaters' and there are 'stoths'. The flying phalanger is comparable with the flying squirrels and Australia even has its own 'marsupial mole.

Marsupials and placentals are all mammals and do possess a common ancestry in the not too remote past. Convergence can, however, produce similarity between completely unrelated creatures. Thus insects and birds are far removed from one another yet at a glance it is very difficult to tell the difference between a humming birds and a humming bird hawk moth in flight. Both are similar in size and both live off nectar in flowers and have converged in their hovering flight and their feeding procedures.