GLOSSARY 443 



Cephalization. The degree of organization obtained by the anterior or 



head end of the animal body. 

 Cephalothorax. In Crustacea the body is marked off into two main 



regions, the cephalothorax, consisting of the head and thorax, and 



the abdomen. 

 Chemical compound. A substance made up of molecules of the same 



sort. 

 Chemotherapy. Treatment of bacterial disease by means of chemical 



agents. 

 Chemotropism. Response to chemicals. 

 Chitin. a tough organic substance that forms the chief ingredient of 



exoskeletons of invertebrates, notably Arthropoda. 

 Chlorophyll. The green coloring matter in plants. It is a catalytic 



agent that in the presence of energy from sunlight causes carbon 



dioxide and water to unite to form carbohydrate. 

 Chloroplastid. a plastid containing chlorophyll. 

 Choanocyte. a type of cell occurring in members of the phylum Pori- 



fera. The cell is an elongated oval shape; on one end is a collar 



in the centre of which is a beating flagellum. 

 Chromatin. A substance characteristic of the nucleus of cells. It stains 



heavily with certain dyes and is known to be largely composed of 



nucleo-proteins. In this substance reside the inheritable characters. 

 Chromosome. A definite nuclear unit composed of chromatin. Within a 



species the chromosomes are always identical in number and shape. 

 Chyle. The food as it leaves the small intestine after digestion and ab- 

 sorption have occurred. 

 Chyme. The food as it leaves the stomach after stomach digestion. 

 Cilia. Motile protoplasmic fibres on the surfaces of some Protozoa and 



on the cells which form some epithelial linings in the Metazoa. 



In the Protozoa they constitute organs of locomotion. On metazoan 



epithelial cells their motion causes currents in the overlying liquids. 

 Cleavage. The early stage of development of the egg during which the 



first cell divisions occur. 

 Cloaca. A cavity of the posterior portion of the alimentary canal in 



fishes, Amphibia, reptiles and birds, that receives both the contents 



of the intestine and the excretory products of the kidney. 

 Cnidoblast. An ectodermal cell peculiar to the members of the phylum 



Coelenterata, It contains a barb, the nematocyst, and has a sensitive 



