372 ANIMAL BIOLOGY. 



distinction to the visceral (or splanchnic) layer. Parieto-frontal, also 

 called fr onto -parietal, a bone in the skull of the frog. 



Parthenogenesis. See Ovum. 



Periostracum. Also called, as in the crayfish, epiostracum, the outer 

 layer of the shell in the mussel. 



Physiology. See Morphology. 



Placenta. A vascular mammalian structure bringing the embryo into 

 relation with the uterus of the mother. Villi on the allantoic chorion 

 fit into depressions or crypts of the uterine wall, the conjoint structure 

 forming the placenta. If the foetal structures separate at birth from 

 the maternal mucous membrane, the placenta is non-deciduate ; if the 

 maternal mucous membrane is torn away with the foetal structures 

 and delivered as the after-birth, it is deciduate. According to the 

 arrangement of the villi the placenta is discoidal (rabbit), dome shaped, 

 zonary, or diffuse. 



Pleura. (1) The membrane that covers the mammalian lungs. (2) The 

 lateral edges of the exoskeleton of the crayfish. 



Portal. A portal vein is one which instead of carrying the blood directly to 

 the heart carries it to some organ or gland within which it breaks up 

 into a capillary plexus. All vertebrates have a hepatic-portal system, 

 with a capillary plexus in the liver. The frog has also a renal portal 

 system, with a capillary plexus in the kidney. 



Process. A projecting portion proceeding from some cell, organ, or part 

 of the body; e.g. transverse process of vertebra, pseudopodial processes 

 of amoeba. 



Procoelous. Of vertebrae, the centra of which are concave in front ; amphi- 

 ccelous are concave at both ends ; opisthocodous concave posteriorly. 



Proctodseum. That part of the alimentary canal which arises in develop- 

 ment as a posterior epiblastic invagination, giving rise in the cray- 

 fish, e.g. to the hind-gut. 



Prcecoces. See Al trices. 



Protovertebrse. A term sometimes used for mesoblastic somites. 



Protozoa. See Metazoa. Also called cytozoa. 



Protoplasm. The elementary organic basis of living matter, sometimes 

 termed sarcode. 



Proximal See Distal. 



Racemose. A term applied to a gland in which the lobules contain a 



number of acini opening in clusters into the extremities of a branched 



duct. 

 Eamus. A branch or division, (1) of a nerve, the spinal nerves, e.g. have 



dorsal and ventral rami ; (2) of the lower jaw, each lateral mandible 



being termed a ramus. 

 Rejuvenescence. A term applied to the added energy of a protozoon after 



conjugation. Also termed adjuvenescencc. 



