GLOSSARY 379 



Dyad. A double body formed by the division of a tetrad into two parts. Its two 

 parts may be derived from the same chromosome or from different chromosomes. 



Echinoderm (e ki' no derm). One of the Echinodermata. 



Echinodermata (e ki' no der' ma to). The phylum of animals including the starfishes, 



sea urchins, sea cucumbers, brittle stars, etc. For definition see Chap. 19. 

 Echinoidea (ek' i noi' de a). A class of Echinodermata, comprising the sea urchins, 



sand dollars, and heart urchins. For definition see Chap. 19. 

 Echinorhynchus (e ki' no ring' kus). An Acanthocephalan worm. 

 Ecology {e koV oji). The branch of biology dealing with the relation of animals or 



plants to their environment. 

 Ectoderm. The outer layer of cells of a gastrula, or the representative of this layer 



in later stages. 

 Ectosarc. The outer layer of protoplasm in cells in which the outer and inner proto- 

 plasm differ distinctly in structure, as in Amoeba. 

 Edaphosaurus (e daf o saw' rus). An extinct lizardlike reptile bearing a spiny fin 



on its back, from Permocarboniferous rocks of North America. 

 Effector. A structure specialized for some specific response; also the nerve carrying 



impulses to such a structure. 

 Efferent. Leading from; said of nerve fibers which conduct impulses away from the 



central nervous system. 

 Elasmobranchii (e laz' mo brang' ki i). A class of Vertebrata comprising the sharks, 



skates, rays, torpedoes, and chimaeras. For definition see Chap. 19. 

 Electrolysis (e lek troV i sis). Decomposition of an ionized substance in solution by 



passing an electric current through the solution. 

 Electrolyte. A substance which, because it ionizes, is in solution capable of conduct- 

 ing an electric current and of being decomposed by the current. 

 Electron. A unit of negative electric charge entering into the composition of atoms. 

 Element. One of the approximately 90 primary forms in which matter exists. 

 Elephas {eV e fas). A genus of animals including living elephants and their fossil 



relatives of Pleistocene time. 

 Elodea {eV o de' a). A genus of aquatic plants. 

 Embryo. An undeveloped animal while still in the egg membrane or in the maternal 



uterus. 

 Embryology. The science which deals with the development of the embryo, or 



young stages, of animals or plants. 

 Embryonic. Pertaining to an embryo. 

 Empedocles {em ped' o kleez). Greek philosopher and poet, born in Sicily. Lived 



about 490-430 B.C. 

 Emulsion. A mixture of two liquids or semiliquid substances, neither one soluble 



in the other, the one being in the form of separate droplets suspended in the 



other. 

 Emulsoid. A mixture consisting of a liquid in which are distributed particles of a 



liquid or semisolid substance which are exceedingly minute yet larger than 



molecules. 

 Emys (e' mis). A genus of turtles of the family Testudinidae. 

 Enamel. The very hard, polished calcareous substance forming the surface layer or 



internal plates in the teeth of mammals. 

 Endocrine secretion. A secretion which must leave the gland by diffusion, not 



through a duct. 

 Endoderm. The inner layer of cells of a gastrula, or the representative of this layer 



in later stages. 



