218 GLOSSARY. 



Peri-pharyngeal bands. The ciliated bands that connect the endostyle 



and dorsal lamina of an ascidian around the mouth. 

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



derms). 



Perisarc. The secreted outer covering of a hydroid. 

 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 coelom. 

 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. 

 Pleurobrancji. 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 coalenterate. 

 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 sometimes 



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). 



