GLOSSARY 495 



Cheliped. The first thoracic appendages of the Crayfish and its allies. 



The 'pincer.' 

 Chemotropism. A simple orienting response, either positive or negative, 



to chemical stimuli; e.g., of Paramecium or sperm. Chemotaxis. 

 Chloragogen Cells. Outer layer of intestine of Earthworm. Probably 



excretory in function. 

 Chlorophyll. The characteristic green coloring matter of plants, through 



which photosynthesis takes place. Comprises two pigments. 

 Chloroplasts. The special cytoplasmic bodies containing chlorophyll. 

 Cholesterol. A complex monohydric alcohol of the lipid series. Closely 



related to vitamin D, certain hormones, and cancer-producing sub- 

 stances. See Lipids. 

 Chordate. An animal whose primary axial skeleton consists temporarily 



or permanently of a notochord. All Vertebrates are Chordates, but the 



lowest Chordates are not Vertebrates. See Appendix I. 

 Chorion. External embryonic membrane of Mammals. 

 Chromatin. A deeply staining substance characteristic of the nucleus, 



forming chromosomes, etc. See Germ Plasm. 

 Chromosome. One of the deeply staining bodies into which the chromatin 



of the nucleus becomes visibly resolved during mitosis. A linkage 



group of genes. See Germ Plasm. 

 Cilia. Delicate protoplasmic projections from a cell, which lash in unison 



and propel the cell in the water (e.g., Paramecium), or move particles 



over the cell surface (e.g., cells lining various tubes in multicellular forms). 

 Class. In classification, a main subdivision of a phylum. See Order. 

 Cleavage. Cell divisions which transform the egg into the blastula stage 



during development. 

 Cloaca. A cavity at the posterior end of the Vertebrate body, into which 



the intestine, urinary, and reproductive ducts open. Not present in 



most Mammals. 

 Cochlea. The portion of the ear, in communication with the sacculus, 



which is the essential organ of hearing in the higher Vertebrates. 

 Coelom. The body cavity, enclosed by tissue of mesodermal origin. 

 Coelomate. Possessing a coelom, or body cavity; as in all the chief 



groups of animals above the Coelenterates. 

 Coelomic Epithelium. See Peritoneum. 



Coenosarc. Tissue of the tubular branches of a Hydroid; e.g., Obelia. 

 Collar Cells. Cells with cytoplasmic flange, or collar, surrounding the 



base of the flagellum. Represented by certain Protozoa, and in the 



gastral epithelium of Sponges. 

 Colloid. A state of matter in which a substance is finely divided into 



particles larger than one molecule and suspended in another substance, 



semi-fluid or fluid. 



