Cline. Continuous gradation of form or gene differences in a populations of a species, correlated with its geographical or ecological distribution.

Clisere. A succession of climaxes in an area as result of climatic changes.

Clitellum. Saddle-like region of some Annelida (Oligachaeta, Hirudinae). Prominent in sexually mature animals. Contains glands which secrete a mucus sheath around copulating animals thus binding them temporarily together, and a cocoon in which fertilization and development of eggs occur. cocoon in which fertilization and development of eggs occur.

Clitoris. Homologue of penis in female mammal.

Cloaca.
Posterior part of gut of most vertebrates into which kidney and reproductive ducts open; in such cases there is only one posterior opening to the body, the cloacal aperture. instead of (as in most mammals) separate anus and urinogenital openings. Also terminal part of intestine in some invertebrates, e.g. sea cucumbers.

This converts prothrombin in the blood to its enzymically active from thrambin. Thrambin catalyses the formation of insoluble fibrin from soluble fibrinogen; the fibrin forms a fibrous network in which blood cells become enmeshed, producing a clot. See also blood clotting.

Clustering methods. The means by which groups of related of similar species are arranged in species groups or into higher taxonomic groupings.

Clypeus. The cuticular plate on the head of an insect above the labium.

Cnidaria.
A phylum of radially symmetrical diploblastic animals with tissue-grade of organization and stinging-cells or thread cells present for offence, defence and feeding. Mouth is present, polymorphism (medusoid and polypoid zooids) seen in many examples; also called Coelenterata, includes a free swimming planula larva in the life-cycle; and includes 3 classes, viz. Hydrozoa, Scyphoyzoa and Anthozoa (Actinozoa).