190 GLOSSARY. 



Peristaltic. The motion caused by the successive contraction of the 



muscle fibers in the walls of a tube. 

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



derms). 

 Peristomium. The somite of an annelid that bears the mouth. 

 Peritoneum. The membrane that lines the ccelom. 

 Per-radial canals. The canals of a medusa that lie opposite the corners 



of the mouth. 

 Pharynx. An anterior division of the alimentary canal. 

 Planula. A young coelenterate embryo. 

 Pleopod. An abdominal appendage of a crustacean. 

 Pleurobranch. A gill of a crustacean that is borne on the body-wall. 

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

 Podobranch. A gill of a crustacean that is borne on the basal joint of 



an appendage. 

 Polymorphism. Many distinct forms of individuals. 

 Polyp. An individual of a hydroid stage of a coelenterate. 

 Pore. A small opening. 

 Post-cava. A blood-vessel that leads to the heart from the posterior 



portion of the body. See squid. 

 Posterior. Hinder; anal end. 

 Pre-cava. A blood-vessel that leads to the heart from the anterior end 



of the body. See squid. 

 Primary mesentery. One of the vertical muscular partitions that extends 



from the body-wall- to the esophagus of an actinozoan. 

 Proboscis. Applied to various tube-like organs around the head that are 



usually capable of being everted or protruded. 

 Proglottid. One of the pieces that compose the body of a cestode. 

 Propod. The next to the last segment, sixth, of a typical crustacean 



limb. 

 Prosopyle. One of the pores through which water passes from an incur- 



rent to a radial canal of a sponge. 

 Prostomium. The anterior process that overhangs the mouth of an 



annelid. 

 Prothorax. Anterior division of the thorax of an insect. 

 Protomerite. The anterior part of Gregarina. 

 Protoplasm. Cell substance; living matter. 

 Protopod. The first two segments of a crustacean limb (the protopod 



bears the exopod and endopod). 

 Proximal. Toward the origin or attachment. 

 Pseudopodimn. One of the changeable protoplasmic projections of the 



Sarcodina. 

 Pyloric division of stomach. Posterior or second division. 



