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 Hydromedusae, 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. 



Ileum : 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 lamellae of a 

 lamellibranch's gill ; mussel, 93 ; clam, 107. 



Intermediate host : the animal which harbors the larval form of a para- 

 site, 84. 



