Crossing Over - Term used to describe the breaking and rejoining of chromosomes that sometimes occurs during cell reproduction.

Crossopterygii - (=Choanichthyes) Class of fishes including the coelacanth (Latimeria), the modern lung fishes, and many fossil forms. They are bony fishes but they differ from the typical modern fishes in that the paired fins contain a flesty lobe and a bony skeleton. Most of the crossopterygians also have internal nasalopenings (choanae) feature leading to the alternative name of Choanichthyes. The possession of internal nares suggests that the ancient crossopterygians could breather air and it seems certain that the land living vertebrates evolved from this group of fishes.

Crustacea - A large class of the phylum Arthropoda including crabs; waterf1eas, and woodlice. All have at least at some stage in their life, two pairs of antennae, the first pair of which are called antennules. The majority of crustaceans live in water and breathe by means of gills which are modified limbs or parts of limbs. The cuticle is often strengthened by a deposit of calcium carbonate and it may be developed as a large shield (carapace) covering the front part of the body. The most primitive members of the class have many pairs of similar limbs but there is a tendency in the more advanced forms to specialisation of the limbs and a reduction in the number of body segments. Most of the limbs are biramous, the two branches often performing different functions.

There are six sub classes. The Branchiopoda are free living and normally have many pairs of broad, bristle fringed limbs on the body. The limbs act as gills and also filter food particles from the water. In many cases they also propel the animal through the water but when, as in Daphnia, the water flea, the trunk limbs are enclosed by the shell, the head limbs are used for movement. Sub class Ostracoda contains tiny animals whose carapace develops into a bivalve shell round the whole body. The feathery antennae and antennules move the animal through the water and also sweep food particles into the mouth. Sub class Copepoda contains both free and parasitic forms, the latter often without limbs.

A typical example is Cyclops found in almost any stretch of fresh water. There is no carapace and no limbs on the abdomen. Swimming is performed by the antennae and antennules and also by the thoracic limbs. Members of the sub-class Branchiura are parasites on fish but they are able to swim from one fish to another by means of the four pairs of thoracic limbs. Sub class Cirripedia includes the barnacles sessile, sometimes parasitic creatures very unlike typical curstacea ns.

The trunk limbs of barnacles are used to 'comb' food particles from the water. The sub-class Malacostraca is a large one containing wide range of forms. Typically there are six head segments, eight thoracic segments, and six abdominal ones. A carapace encloses the sides of the thorax and the limbs are modified for walking swimming, food catching, etc. Unlike those of all other crustaceans, the antennules are biramous, Members of the sub-class include crads, lobsters, shrimps, woodlice (the only truly landliving crustaceans), and sand hoppers.

Cryptic Coloration - Colouring or patterning that renders its owner less visible against its natural background. There are several ways in which this is achieved.

Cryptozoic - (1). Living in secluded places. (2) A division of geological time.

Ctenophora - A phylum of free-living solitary animals, probably related to the coeltenterates, although there is neither polyp nor medusa. Known as sea gooseberries or comb jellies, the ctenophores swim in the surface layers of the sea by means of cilia. There are two classes Tentaculata, with tentacles, and Nuda, without tentacles. None has stinging cells.

Cuticle - Non-cellular covering layer found in most invertebrates. It is secreted by the epidermis and is mainly proteid (e.g) chitin although frequently hardened by calcium salts and other substances.

Cyclostomata - Class of vertebrates containing the lampreys and hagfishes the only living jawless vertebrates. They are eel like creatures without paired fins and with circular gill slits. They feed on living or dead fish by rasping away the flesh with their mouths. The alimentary canal is very simple. Hagfishes are entirely marine but some lampreys live in fresh water.

Cytology - The study of cells.

Cytoplasm - The fluid conten of cells other than that contained in the nucleus.