218 INVERTEBRATE ZOOLOGY 
Periopods: the thoracic appendages posterior to the maxillipeds in crus. 
taceans, 33. 
Periostracum: the outer layer of the molluscan shell; mussel, 91; clam, 
105; snail, 115. 
Periphery: the outer surface of a body. 
Periproct: the region immediately around the anus, 150. 
Perisarc: the cuticular outer covering of a hydroid; tubularian, 166; cam- 
panularian, 170. 
Peristome: a membrane surrounding the mouth in echinoderms; starfish, 
143; sea urchin, 149. 
Peristomium: the posterior portion of the head in most annelids, consisting 
of the metastomium and the anterior somites of the metasoma, 62. 
Peritoneum : the membrane lining the body-cavity. 
Pharynx: the division of the alimentary tract immediately back of the 
mouth; grasshopper, 15; Nereis, 64; earthworm, 71; Bugula, 86; 
snail, 117; squid, 1381; Molgula, 136. 
Plankton: a collective term referring to all small forms of life in the 
surface waters of the sea or of fresh water. 
Pleopod: an abdominal appendage in crustaceans, 32. 
Pleurobranch: a gill attached to the body-wall in crustaceans; crayfish or 
lobster, 35; crab, 438. 
Pleurum: the lateral surface of the thorax in insects; wasp, 3; grass- 
hopper, 10. 
Podical plates: paired sclerites at the posterior end of the abdomen in 
certain insects, 11. 
Podobranch: a gill attached to the leg in crustaceans, 35. 
Polian vesicle: a sac extending from the ring canal in echinoderms, 
157. 
Polyp: a sessile individual in the Cnidaria; Hydra, 159; tubularian, 163; 
campanularian, 169; Gonionemus, 177. 
Polypide: the soft parts of a bryozoan, 85. 
Posterior: at or towards the hinder end of an animal. 
Proboscis: a prehensile organ in certain animals, usually a portion of the 
pharynx; Nereis, 64; planarian, 76: the beak-like mouth-parts of 
certain insects, 7. 
Proglottid : a tapeworm segment, 80. 
Prosoma: the primitive head of annelids made up of the prostomium and 
the metastomium, 62, 68. 
Prostate gland: the gland which secretes the fluid in which the spermatozoa 
are suspended ; grasshopper, 17; squid, 132. 
