GLOSSARY 213 
Heart: a muscular tube-like or sac-like organ which propels the blood; 
grasshopper, 14; spider, 26; crayfish or lobster, 38; crab, 44; Daphnia, 
57; earthworm, 70; mussel, 94 ; oyster, 101; clam, 108; snail, 116; 
squid, 129; Molgula, 137; starfish, 148; sea urchin, 154. 
Hemimetabolic: larval development with incomplete metamorphosis in 
insects. 
Hermaphroditic: having the two sexes united in one animal; earthworm, 
72; planarian, 77; tapeworm, 81; Bryozoa, 87; snail, 120; Molgula, 
137; Hydra, 162. 
Hinge ligament: the flexible portion of a bivalve shell which joins the 
two valves; mussel, 89; oyster, 99; clam, 103. 
Holometabolic: insects having a complete metamorphosis. 
Homologous: having had a similar origin. 
Host: the animal which harbors a parasite, 80. 
Hydranth: a feeding polyp in a hydroid colony, 164, 170. 
Hydrocaulus: the stem of a hydroid colony, 164, 169. 
Hydroid: the sessile, asexual generation of the Hydromeduse, 163, 169. 
Hydrorhiza: the root-like projections of a hydroid colony by which it is 
attached, 164, 169. 
Hydrotheca: the cuticular outer covering of the hydranth in campanularian 
hydroids, 170. 
Hypodermis: the cellular layer which forms the inner portion of the integu- 
ment of most invertebrates; crayfish or lobster, 36; earthworm, 74. 
Hypopharynx: a median projection from the ventral wall of the pharynx 
in insects — in many insects an important mouth-part, 13. 
Hypophysis: a ventral projection of the brain in vertebrates, 138. 
Hypostome: the projection of a hydroid’s body which bears the mouth; 
Hydra, 160; campanularian, 170. 
Neum: a division of the intestine in insects, 15. 
Imago: a holometabolic insect which has completed its metamorphosis ; 
an adult insect. 
Integument: the outer covering of an animal; in most invertebrates it 
consists of an outer cuticula and an inner hypodermis. 
Interfilamentary connections: cross-ridges which join the gill-filaments in 
lamellibranchs; mussel, 94; clam, 108. 
Interlamellar partitions: vertical walls which join the two lamelle of a 
lamellibranch’s gill; mussel, 93; clam, 107. 
Intermediate host: the animal which harbors the larval form of a para- 
site, 84. 
