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, 131; 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, 43. 

 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. 



