37G PRINCIPLES OF ANIMAL BIOLOGY 



in the somatic cells, and in germ cells before maturation, in bisexual animals. Cf. 

 haploid. 

 Diplozoon {dip' lo zo' on). A trematode worm parasitic on the gills of fishes. 



Dipnoi (rftp' 7W i). A subclass of Pisces, fishes with an airbladder functioning as a 



lung; the lungfishes. 

 Dissosteira {dis' so sti' ra). A genus of grasshoppers. 

 Division of labor {di vi' zhun ov la' her). Distribution of functions among cells, or 



organs, or individuals. 

 Dominant {dom' i nant). Receiving expression when only one determining gene is 



present, and in the presence of the gene for a contrasted recessive character; said 



of inherited characters that are exhibited by heterozygotes. 



Donacia {do na' shi a). A genus of aquatic beetles. 



Dorsal {dor' sal). Pertaining to the back ; hence, usually, upper. 



Dorsal aorta {dor' sal a or' ta). A large artery formed, in fishes, by the union of 

 vessels coming from the gills, and passing backward in the dorsal region. 



Dorsal root {dor' sal root'). The dorsal one of two roots by which a spinal nerve is 

 connected with the spinal cord. Its fibers are sensory in function. 



Double refraction {di(b' 'I refrnk' shun). The separation of a beam of light into two 

 rays as it passes through a refracting medium. 



Drosophila {dro sof i la). A genus of flies, of which the fruitfly (D. melanogaster, 

 mel' a no gas' ter) is a common species. 



Dujardin, Felix {dii zhar daisi'). French naturalist, 1801-1860. 



Duodenum {du' o de' num). The first of three divisions of the small intestine. 



Dyad {di' ad). A double body formed by the division of a tetrad into two parts. It 

 may consist of two halves of the same chromosome or of two halves of different 

 chromosomes. 



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



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

 sea urchins, sea cucumbers, brittle stars, etc. For definition see Chapter XII. 



Echinoidea {ek' i noi' de a). A class of Echinodermata, comprising the seaurchins, 

 sand-dollars, and heart-urchins. For definition see Chapter XII. 



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

 plants to their environment. 



Ectoderm {ek' to derm). The outer layer of cells of a gastrula, or the representative 

 of this layer in later stages. 



Ectoparasite {ek' to par' a site). A parasite that is attached to the outside of the body 

 of its host. 



Ectosarc {ek' to sark). The outer layer of protoplasm in cells in which the outer and 

 inner protoplasm differ distinctly in structure, as in Amoeba. 



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

 on its back, from Permo-Carboniferous rocks of North America. 



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

 skates, rays, torpedoes, and chima;ras. For definition see Chapter XII. 



Electrolyte (e lek' iro lite). A substance wliich in solution is capable of conducting an 

 electric current and ot being decomposed by the current. 



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

 relatives of Pleistocene time. 



