122 NATURAL HISTORY 



Polyp — a separate zooid of a colonial animal. 

 Prehensile — adapted for holding. 



Premolars — bicuspid teeth between canine and molar teeth. 

 Proboscis — any of various tubular prolongations of the head of animals ; mus- 

 cular protrusible part of the alimentary canal in certain worms. 

 Procoelus — with concave anterior face. 



Prothallus — a small, thin, gametophytic mass of tissue developed from spores 

 of ferns. 



Prothorax — anterior thoracic segment of arthropods. 



Protractile — capable of being thrust out. 



Pseudopodia — protrusions of protoplasm (false feet) serving for locomotion and 

 prehension in protozoa. 



Pygostyle — an upturned compressed bone at end of vertebral column of birds, 

 formed by fusion of caudal vertebrae. 



Pyloric caecum — a blind diverticulum or pouch in the pyloric region. 



Pylorus — the aperture between the stomach and the small intestine. 



Pyrenoid — a colorless plastid of lower plants, a center of starch formation. 



Quadrate bone — the bone with which the lower jaw articulates with the cranium 

 in some forms. 



Radially symmetrical — having similar parts arranged on either side of a central 



axis. 



Reproduction — the process by which organisms produce off .spring. In asexual 

 reproduction a new organism is formed by the separation of a cell or cells 

 from a single parent; in sexual reproduction two cells from two plants or 

 two animals of different sexes join together to form a new individual. 



Rhizoids — rootlike organs. 



Rhomboidal — shaped more or less like an equilateral parallelogram, having its 

 angles oblique. 



Rodent — animal with a habit of gnawing or nibbling. 



Sap cavity — a vacuole, filled with water and dissolved substances in mature, 

 live plant cells. 



Saprophytes — organisms which live on dead and decaying organic matter. 



Scale — a flat, small, platelike external structure, dermal or epidermal. 



Schizogony — repeated division of the nucleus without immediate cell division. 



Sedentary — not free-living; animals attached by a base to some substratum. 



Segmentation — the division or splitting into segments or portions ; cleavage of 

 an ovum. 



Sessile — stationary or attached, opposite of free-living or motile. 



Setae — bristlelike structures. 



Siphon — a tube through which water may pass into and out from the mantle 



cavity of a mollusc. 

 Sperm — male sex-cell. 



Spicules — siliceous or calcareous secreted skeletal structures of sponges. 

 Spiral valve — a spiral infolding of intestinal wall. 

 Spongin — material of skeletal fibers of certain sponges. 

 Sporangium — a sac containing spores. 

 Spore — a type of reproductive cell, usually asexual, with a protective covering 



enabling it to survive unfavorable environmental conditions. 

 Sporophyll — a sporangium-bearing leaf of ferns. 

 Sporophjrte — spore-bearing stage in the life cycle of a plant. 

 Stapes — stirrup-shaped innermost bone of middle ear of mammals. 



