680 Glossary 



Pelvic girdle — a group of bones that supports the hindlimbs. 



Periosteum — a tough membrane surrounding a bone. 



Pericardium — a sac of serous membrane surrounding the heart. 



Peristalsis — the waveHke movements of the digestive tract by which food is 

 moved along. 



Peritoneum — serous membrane Hning the body cavity and surrounding the vis- 

 ceral organs in the eucoelomates. 



Peripheral — pertaining to the outer surface. 



Pharynx — the portion of the digestive tract between the cavity of the mouth and 

 the esophagus. 



Phenotype — visible characters of an individual as contrasted with his genetic 

 composition. 



Photophore — an organ that emits light. 



Phylum — in classification, a category more inclusive than the class. 



Placenta — the vascular organ of attachment of the developing young to the mother. 



Plankton — the group of floating or weakly swimming organisms in a body of water. 



Planula — larva of coelenterates. 



Plasma — the liquid portion of the blood. 



Platelets — the formed element of the blood concerned with clotting. 



Platyhelminthes — the phylum of animals that contains the flatworms. 



Pleura — membrane surrounding the lungs. 



Plexus — a network. 



Poikilothermic — having a variable body temperature. 



Polar body — one of the bodies containing chromosomes that is cast off during 

 oogenesis. 



Polyp — the attached stage of certain invertebrates, especially the coelenterates. 



Pons — a band of nerve fibers connecting the cerebellar hemispheres. 



Porifera — the phylum of animals that contains the sponges. 



Portal system — a system of blood vessels that begins and ends in capillaries. 



Posterior — away from the head or front. 



Premolar — a type of tooth adapted for grinding. 



Process — a projection on a structure. 



Protonepliridial system — the excretory sy.stem that has flame cells as the filtra- 

 tion unit. 



Protoplasm — the basic substance that composes all organisms. 



Protoplast — the protoplasmic contents of a cell. 



Protozoa — the phylum of animals containing the single-celled forms. 



Protein — one of the basic food substances and also an essential constituent of 

 all living organisms. 



Proximal — toward the point of attachment to the main body mass. 



Pseudocoel — a body cavity not lined by peritoneum. 



Pseudopodium — a protoplasmic extension from the main mass of the cell. 



Pulmonary — pertaining to the lungs. 



Pylorus — the muscular valve separating the stomach from the small intestine. 



