486 THE PINEAL ORGAN 



the single terminal vesicle of the parietal organ of vertebrates." He 

 believed that it contains in its walls two of the four retinal placodes, 

 belonging to the undivided terminal vesicle. It corresponds to the 

 parietal eye of Petromyzon (Studnicka). 



Edriophthalmata (Gr. hedraios, sitting ; ophthalmos, eye) : a division of the 

 Crustacea having sessile eyes, not supported on a stalk. It includes the 

 Amphipoda, e.g. sandhopper (Talitrus), and Isopoda, e.g. fresh-water 

 shrimp (Gammariis). 



Elasmobranchii (Gr. elasma, plate ; branchia, gill) : an order of fishes including 

 the sharks and rays, having plate-like gills. 



Endochrome Cells (Gr. endon, within ; chroma, colour) : pigmented cells 

 of the retina in which the pigment granules are deposited in the epithelial 

 cells composing the retina. The pigment may be deposited in one part 

 of a sensory or visual cell or in a specialized epithelial pigment cell. 



Endocrine (Gr. endon, within ; krind, separate) : adjective used to denote the 

 function of internal secretion. 



Endoparietal eye (Gr. endon, within ; L. paries, wall) : term applied by 

 Patten to " the inner of the two vesicles produced by constriction of 

 the terminal vesicle of the parietal organ of vertebrates." He believed 

 that it contains in its walls two of the four retinal placodes belonging to 

 the undivided terminal vesicle. It corresponds to the " parapineal 

 organ " of Petromyzon (Studnicka). 



Endothelium (Gr. endon, within ; thele, nipple) : a layer of cells lining a vessel 

 or vascular space, or the epithelial lining of a serous membrane. 



Entoderm (Gr. entos, within ; derma, skin) : the inner layer of embryonic 

 cells, also called endoderm and hypoblast. 



Entomostraca (Gr. entomos, incision, division ; ostrakon, shell) : a sub-class 

 of the Crustacea in which the shell is divided into segments. It com- 

 prises the orders Phyllopoda, Cladocera, Ostracoda, Copepoda. 



Entomostracan eye : the name given to the median eye-spot which is 

 commonly found in species belonging to the four orders which are 

 included in the subclass Entomostraca. It usually consists of three 

 single eyes joined into a triple eye, one segment of which is median and 

 ventral, the remaining segments being lateral and dorsal. The median 

 segment appears to be formed by the fusion of two primarily bilatera' 

 eyes. 



Epiblast (Gr. epi, upon ; blastos, germ) : the outer or upper layer of cells in 

 the embryo. See Ectoderm. 



Epiphysis cerebri (Gr. epi, upon ; phusis, originate or grow from) : the diverti- 

 culum from the roof of the diencephalon, which gives rise to the pineal 

 body, pineal sac, or parietal sense-organ. 



Epithelium (Gr. epi, upon ; thele, nipple) : a layer of cells covering a surface 

 or lining a duct, canal, or vessel. It covers the nipple-shaped dermal 

 papilla; of the skin. 



Epizoa (Gr. epi, upon ; zoon, animal) : (i) animals which are parasitic on other 

 animals ; (2) a subclass of the crustaceans in which the adult animal 

 is usually fixed to the gills or exterior of a fish or other animal. The 



