380 PRINCIPLES OF ANIMAL BIOLOGY 



Gastropoda {gas trop' o da). A class of Mollusca including the snails and slugs, 

 moUusks whose bilateral symmetry is often obscured by a coiled body and shell. 



Gastrovascular {gas' tro vas' ku ler). Serving the functions of digestion and circu- 

 lation. 



Gastro vascular cavity {gas' tro vas' ku ler kav' i ti). See ccelenteron. 



Gastrula {gas' tru la). An early developmental stage, formed from a blastula by the 

 invagination of the vegetative pole of the latter. The gastrula consists of two layers 

 of cells (ectoderm and endoderm) surrounding a cavity which communicates with 

 the exterior. 



Gastrulation {gas' tru la' shun). The invagination of the vegetative pole of a blastula 

 into the blastocoele. 



Gelatin {jeV a tin). A jelly-like substance obtained by boiling from cartilage, bone, 

 tendon, hgament, connective tissue, etc. 



Gemmule {jem' mule). A group of cells forming a reproductive body in freshwater 

 sponges. 



Gene {jeen). Something in a germ cell or other cell which is responsible for a heredi- 

 tary characteristic. Also called factor. 



Generic {je ner' ic). Pertaining to a genus. 



Genetics (je net' iks). The science of heredity, variation, sex determination, and 



related phenomena. 

 Genital ijen' i tal). Concerned with reproduction. 

 Genus {je' nus) {pi., genera, jen' e ra). A group of species having so many structural 



features alike that they must be regarded as having sprung from common ancestry; 



a group of lower rank than the family. 

 Geofifroy-Saint-Hilaire, Etienne. {zhofrwa'sa^telair'). French naturalist, 1772- 



1844. 

 Geotaxis {je' o tak' sis). An orientation of an organism with reference to the stimulus 



of gravity. 

 Geotropism {je of ro piz'm). The response of an organism to the stimulus of 



gravity, 

 Gephyrea {jefeer' i a). A group of worm-like animals of doubtful rank and relation- 

 ships. They have sometimes been referred to the Annelida. 

 Germ cell {jerm' seV). A cell capable of reproduction, or of sharing in reproduction, 



as contrasted with the somatic or body cells which are sterile. 

 Germinal {jer' mi nal). Relating to the germ cells; said of epithelium, for example. 

 Gill {gil). A structure having a surface enlarged usually by branching or folding, 



which serves a respiratory function. 

 Gill bar {giV bar'). The tissue between two gill clefts. 

 Gill cleft {giV kleft'). One of several openings from the pharynx to the sides of the 



neck or head of a vertebrate embryo or adult; derived from a gill pouch. Also 



called gill slit. 

 Gill pouch {gil' pouch'). One of several evaginations from the sides of the anterior 



part of the digestive tract in the embryos of vertebrate animals. In some animals 



they break open to the outside, becoming gill clefts. 

 Gizzard {giz' zerd). In the earthworm, a thick-walled portion of the alimentary 



tract behind the crop. In birds, the posterior muscular division of the stomach. 



Glacial {gla' shal). Pertaining to glaciers, or to the period when glaciers were com- 

 mon in regions now temperate, namely, late Cenozoic time. 



