380 PRINCIPLES OF ANIMAL BIOLOGY 



Endosarc. The inner mass of protoplasm in cells in which the outer and inner proto- 

 plasm differ in structure. 

 Endoskeleton. A skeleton within the fleshy parts, as in vertebrate animals. 

 Energy. The capacity for performing work. It is kinetic when employed in pro- 



dvicing motion or heat, potential when stored in chemical combination. 

 Enterokinase {en' ter o ki' nase). An enzyme produced in the small intestine and 



serving to convert trypsinogen into trypsin. 

 Enteron. A digestive system open at both ends. 

 Enteropneusta {en' te rop nu' sta). A subphylum of Chordata, wormlike animals, 



of which Balanoglossus and Cephalodiscus are representatives. 

 Entomology. The zoology of insects. 

 Enzyme {en' zime). An organic substance which brings about a chemical reaction 



but is not consumed bj- that reaction. Probably it participates in the reaction 



but is promptly restored. 

 Eocene (e' o seen). Of the earliest Cenozoic and Tertiary time. 

 Eohippus {e' o hip' pus). The earliest known ancestor of the horse, an extinct animal 



of Eocene time. 

 Epidermis. The outer of the two principal layers of the skin. Also an outer layer of 



cells in general. 

 Epistylis (rp' i sti' lis). A genus of colonial ciliated Protozoa, resembling Vorticella. 



E. flavicans {flav' i kanz), one of the species. 

 Epithelial. Pertaining to an epithelium; as epithelial tissues or structures. 

 Epithelium. A layer of cells at the surface of a tissue or organ, or lining a cavity. 

 Epoch. One of the divisions of a period in the geological time scale. 

 Equation division. A division in which chromosomes are duplicated, producing two 



equal cells ; said of one of the divisions of germ cells as contrasted with the other 



or meiotic division. 

 Equatorial. In the plane of a great circle halfway between the poles; said of a cleavage 



plane of an egg. Also, in a middle position in other objects. 

 Equatorial plate. The flattened group of chromosomes on the middle of the spindle 



of a dividing cell. Also, the plane which they approximately occupy. 

 Equus {e' kwus). A genus of animals including the living horse and some of its fossil 



relatives of Pliocene and Pleistocene time. 

 Era. One of the five major divisions of geological time. 

 Erepsin (e rep' sin). A proteolytic enzyme produced in the small intestine. 

 Ergosterol {er goa' ter ol). A substance, chemically a solid alcohol, obtained from 



ergot, a fungus. On irradiation with ultraviolet it possesses strong antirachitic 



properties. 

 Erosion. The wearing away (of rocks) through the action of water and other agencies. 

 Esophagus (e sof a gr(/.s). In the earthworm, a narrow passage leading from the 



pharynx to the crop. In vertebrates, the passage between the pharj-nx and the 



stomach. 

 Estrogen. A hormone or grouj) of liornioiics produced by the follicles of tlie human 



ovary; several other names have been applied to it. 

 Euarctos (// ark' lose). A genus of bears, including the western black l)ear. 

 Eudorina elegans {u' do ri' nn). A species of colonial chlorophyll-bearing organism 



whose cells are imbedded in a spherical jell^ylike mass. 

 Euglena ('/ gle' na). A genus of green flagellate Protozoa. 

 Eustachian tube {u sta' ki an). A passage between the pharynx and the tympanum 



or middle ear. 

 Eutheria {n the' ri a). A subclass of Mammalia comprising tlic A-iviparous mammals. 

 Eutrephoceras in' t re fox' er ns). .\ genius of extinct ceiilialo])ods rcsemhiing Nautilus. 



