394 PRINCIPLES OF ANIMAL BIOLOGY 



Period. One of the divisions of an era in the geological time scale. 



Periodic. Occurring at rather regular intervals; said of migration which depends 



on the seasons or on the age of the migrating animals. 

 Peripheral nervous system. In general, the nerves, collectively ; the nervous system 



aside from the brain and spinal cord or other central cord. 

 Perisarc. The tough sheath surrovmding the stalk and branches of a hydroid. 

 Peritoneum {per' i to ne' urn). A sheet of cells covering the viscera and lining the 



body cavity in many animals. 

 Permeable. Permitting the passage of both liquids and dissolved substances. 

 Permian. Belonging to the close of the Paleozoic era. 

 Petrifaction. The piecemeal substitution of mineral matter for the body substance 



of dead animals or plants. 

 Phalanx {fa' lanks) {pL, phalanges, fa Ian' jeez). Any one of the bones of the fingers 



or toes in vertebrate animals. 

 Pharynx {far' inks). In an earthworm, the thick-walled portion of the digestive 



tract just posterior to the buccal pouch and in front of the esophagus. In verte- 

 brates, the portion of the digestive tract at the back of the mouth, into which the 



gill clefts open. 

 Phoronidea {fo' ro nid' e a). A small group of marine animals, of which Phoronis is 



the onl}'^ genus, of uncertain relationship to other animals. Sometimes placed in 



a phylum with the Bryozoa and Brachiopoda. 

 Photosynthesis. The construction of glucose from carbon dioxide and water by 



the energy of sunlight in the presence of chlorophyll. 

 Phylum. One of a dozen or more major groups into which the animal kingdom is 



divided; in general, the largest group of which it can be said that the members 



are related. 

 Physalia. A very complex colonial coelenterate, one of the siphonophores. 

 Physiology. The branch of biology which deals with the functions of animals and 



plants, and the processes going on in them. 

 Piltdown. A locality in Sussex, England, near wliich primitive human fossils have 



been found. 

 Pineal body {jnn' e al). A structure on the dorsal side of the brain in vertebrate 



animals. Because of its similarity, in development, to the embryonic stages of an 



eye, it is often called the pineal eye and is believed by many to be a vestigial 



sense organ. 

 Pisces {pis' seez). A class of vertebrate animals including the fishes. For definition 



see Chap. 19. 

 Pithecanthropus {pilh' e kan thro' pus). An extinct apelike and manlike animal 



believed to be closely related to the early ancestry of man. 

 Pituitary {pi tu' i ta ri). A glandular organ beneath the brain composed in part of 



nervous tissue. 

 Placenta. A vascular tissue dovetailing into the wall of the uteiiis on one side and 



(connected with the umbilical cord on the other, thus forming an intimate nutritive 



connection betweeen the embryo and the mother in viviparous animals. 

 Planaria. A genus of flatworms, phyhmi Platyhelminthes. 

 Planula {plan' u la). A ciliated larva consisting of a solid elliposidal mass of cells, 



dov(^loped from the fertilized egg of a medusa or similar organism. 

 Plasma. The liquid part of the blood. 

 Plasmodroma. A subphylum of protozoa devoid of cilia. 

 Plastid. One of several kinds of protoplasmic bodies in cells, like the green bodies 



in {)lant cells, which are centers of chemical activity. 

 Plastron. The flat plate of bones on the ventral side of a turtle. 



