GLOSSARY. 289 



Pharynx. A. membranous sac, in which, inter alia, the mouth 



and the oesophagus open. 



Phragmacone. The conical endoskeleton of a Belemnite. 

 Phylogenesis. The development of the race from the lowest to 



the highest forms. 

 Phylogeny. "A branch of biological speculation" which seeks to 



determine the ancestral history of species. 

 Phylum. Hackel's name for a subkingdom. 

 Physoclistous. When the swim-bladder of a fish has no duct. 

 Pilidium. The helmet-shaped larva of certain worms. 

 Pineal gland. A small non-glandular body connected to the 



optic thalami. 

 Pituitary gland. A two-lobed non-glandular body at the base of 



the brain. 



Placenta. A vascular structure formed by the interlacing of the 

 villi of the chorion and the inner membrane of the uterus in 

 pregnancy. 

 Placoid. Applied to bony fish-scales, whether in the form of 



plates, grains, or spines. 

 Plantigrade. When a Mammal walks on the soles of its hind 



feet.' 



Plantula. An accessory joint between the claws of certain in- 

 sects. 



Planula. The locomotive embryo of many Hydroids. 

 Plasma. The fluid part of the blood in which the corpuscles 

 float. "By the evolution of albuminous granules and oil-par- 

 ticles plasma becomes yolk" (Owen). 



Plasmodium. A complex network of protoplasm, only obser- 

 vable in the lowest organisms. 

 Plastide. An independent mass of protoplasm. 

 Plastron. The ventral plate of Chelonia. 

 Picon. The seven abdominal segments or somites of some of the 



lower Crustaceans. 

 Pleopoda. The appendages attached to the abdominal segments 



of some of the lower Crustaceans. 

 Pleura. The serous membrane investing the lungs and reflected 



on the walls of the thorax. 



Pleurodont. When a tooth is attached to the jaw by one of its 

 fangs with the inside of the socket. 



TJ 



