GLOSSARY 697 



Ganglion. An aggregation of nerve cells. 



Gastro vascular space. The central cavity in Cnidaria and Ctenophora. 



Gastrozooid. A feeding zooid of a siphonophore colony. 



Gemmule. A capsule in freshwater sponges containing archeocytes which live 



through the winter and in the spring form the starting point of a new 



sponge-colony. 

 Gastrula. A stage in the development of the embryo in which two cell layers 



are present, ectoderm and entoderm. 

 Gill. An organ for the breathing of air contained in the water. 

 Gill-filament. Ciliated vertical ridges on the surface of the gills of pelecypods. 

 Girdle. An integumental fold in chitons surrounding the shell. 

 Gizzard. A portion of the alimentary tract in certain animals with thickened 



muscular walls in which the food is chewed. 

 Glochidium. The larval form of Anodonta and Unio which lives a parasitic 



life in the skin of fishes. 

 Gnathopod. A grasping claw in amphipods. 

 Gonangium. The cuticular covering or gonotheca of the blastostyle in cam- 



panularian hydroids. 

 Gonosome. The medusoid stage of a hydromedusan. 

 Gonotheca. The cuticular outer covering of the blastostyle. 

 Gonozooid. The reproductive zooid of a siphonophore colony. 



Haemal. Pertaining to the blood system. 



Holler's organ. A sense organ on the anterior pair of legs in certain ticks. 



Head. The anterior body-division of the higher animals. 



Heart. A muscular tubular or saccular organ which propels the blood through 



the arteries. 

 Hectocotylus arm. One of the arms in the male in many cephalopods used to 



transfer sperm to the female. 

 Hermaphroditic. Having the two sexes united in one animal. 

 Hinge ligament. The flexible portion of a bivalve shell which joins the two 



valves. 

 Homologous. Having had a similar origin. 

 Host. The animal which harbors a parasite. 

 Hydranth. An individual feeding-polyp in a hydroid colony. 

 Hydrocladium. Small branches in the Plumulariidae bearing the polyps. 

 Hydrocaulus4 The stem of a hydroid colony. 

 Hydroid. The sessile, asexual generation of the Hydromedusae and Scypho- 



medusae. 

 Hydrorhiza. The root-like projection of a hydroid colony by which it is 



attached. 



Hypodermis. The cellular layer which forms the inner portion of the integu- 

 ment of most invertebrates and secretes the cuticula. 



Hypophysis. A ventral projection of the brain in vertebrates. 



Hypopus. The youthful form of certain mites. 



Hypostome. The projection of a hydroid 's body which bears the mouth. 



Infrdbasals. Calcareous plates between the basals in certain crinoids. 



Infundibulum. A longitudinal fold of the pharyngeal wall of ascidians. 



Insertion plate. The portions of tlie shell of chitons beneath the girdle. 



Integument. The outer covering of an animal; in most invertebrates it con- 

 sists of an outer cuticula and an inner hypodermis. 



Interlamellar partitions. Vertical walls which Join the two lamellae of a pele- 

 cypod 's gill. 



Intermediate host. The animal which harbors the larval form only of a 



parasite. 



