262 GLOSSARY 



Papilla. A small projection. 



Paragnatha. Lamellae behind the mandibles of some Crustacea. 



Parapodium. Typical annelid appendage. Usually one pair per seg- 

 ment. 



Parenchyma. Any soft tissue; specifically, the tissue occupying much 

 of the space between the body wall and digestive tract in a flat 

 worm . 



Pectine. One of a pair of appendages of scorpions. 



Pedicel. The stalk supporting a hydranth or gonozooid. 



Pedicellaria. A minute pincer-like organ which is present on As- 

 teroidea and Echinoidea. 



Pedipalpi. The second pair of appendages in arachnids. They lie 

 on each side of the mouth. 



Peduncle. A short stalk. 



Pelagic. Said of organisms which live at or near the surface of the 

 water. 



Pen. Vestigial internal shell of a cephalopod, as in the squid. 



Penis. Male intromittent organ. 



Pericardium. The membrane surrounding the heart. 



Pereiopod. The walking leg of a crustacean. 



Peripharyngeal bands. The ciliated bands situated in the anterior 

 region of the pharynx and connecting the endostyle with the dorsal 

 lamina in many protochordates. 



Periproct. The region around the anus (especially applied to the 

 echinoderms). 



Perisarc. The chitinous covering of the coenosarc in hydroids. 



Peristalsis. The motion caused by the relaxation and contraction of 

 the muscle fibers in the walls of a tube. 



Peristome. The region around the mouth (especially applied to echino- 

 derms) . 



Peristomium. The first somite of an annelid. In it the mouth lies. 



Peritoneum. The membrane which lines the coelom. 



Per-radial canals. In a medusa, the canals which run from the stomach 

 to the circular canal and which lie opposite the corners of the 

 mouth. 



Pharynx. An anterior division of the alimentaiy canal. 



Planula. A young ciliated, free-swimming coelenterate embryo con- 

 sisting of two cell layers. 



Pleopod. Any abdominal appendage of a crustacean. 



Pleurobranch. A crustacean gill which is borne on the body wall. 



Pleuron. One of the lateral pieces or processes of a somite of an ar- 

 thropod. 



Podobranch. A crustacean gill which is borne on the basal joint of an 

 appendage. 



