_Ciliate_: A protozoan whose motor-apparatus consists of cilia, _i. e._, hairlike protoplasmic projections capable of rapid and coördinated vibratile movement. _Cloaca_: A common passageway through which the intestine, kidneys, and sex organs discharge their products,—it occurs in certain fishes, in amphibia, reptiles, and birds, and in a few mammals. _Coccyx_: Lower extremity of the vertebral column in man. _Colloids_: Insoluble gumlike substances, which will not diffuse through organic membranes. _Commensalism_: The harmonious cohabitation of two organisms belonging to different species, where the relation is not necessarily beneficial nor necessarily harmful to either. _Crossover_: The exchange or reciprocal transfer of whole blocks of genes from one homologous chromosome to the other, which sometimes occurs in synapsis, probably at the strepsinema-stage. _Crystalloids_: Soluble substances, which usually form crystals and readily diffuse through organic membranes. _Cyst_: A protective envelope formed around an organism during period of rest. _Cytode_: The non-nucleated cell hypothecated by Haeckel. _Cyptoplasm_: The cell-body or extranuclear protoplasm of a cell. _Endomixis_: A process of nuclear reorganization among the protozoa, which does not require the coöperation of two cells as in conjugation (amphimixis). _Endoskeleton_: An internal living skeleton providing support and protection (as well as organs of movement, in the bone-levers to which the muscles are attached)—it is characteristic of the vertebrates. _Enzymes_: Organic catalysts, _i. e._, complex chemical substances formed by organisms and serving to accelerate chemical processes taking place in said organisms, _e. g._, the digestive enzymes, which accelerate the hydrolysis of starches, fats, and proteins. _Epigenesis_: Development of the embryo by differentiation of previously undifferentiated protoplasm. _Fats_: Esters of the higher fatty or organic acids (such as stearic, palmitic, and oleic) esterified with the trihydric alcohol glycerine (glycerol). _Gamete_: A reproductive cell specialized for syngamy, _i.e._, for union with a complementary germ cell, their union giving rise to a synthetic cell known as a zygote. _Ganglion_: An aggregate of nerve-cells consisting mainly of neural cell-bodies together with supporting cells. _Ganoids_: Fishes covered with enameled bony scales, and now, for the most part, extinct. _Gene_: A factor or infinitesimal element in a nuclear thread or chromosome, the latter being a linear aggregate of such factors, each having definite specificity and manifesting itself in the external character which develops from it.