GLOSSARY 59 



Segment: A part of the body of an animal shown either by con- 

 strictions on the surface, by appendages, or by internal struc- 

 ture. 



Segmental appendages: Outgrowths from the body such as legs of 

 the centipede which are regularly arranged on different 

 body segments. 



Septum: A partition or dividing membrane. 



Sessile: Attached ariimals as distinguished from free living. 

 Hydra is a sessile form. Also used in the case of organs to 

 signify the absence of a stalk, as: the eyes are sessile, not 

 stalked. 



Setae: Bristles such as project from the body of the earthworm. 



Silicious: Composed of silicon and therefore glasslike. 



Solitary: Animals living alone; used both to distinguish animals 

 not organically connected with each other from those that are 

 organically connected and also to distinguish hermit-like 

 species from those that live in colonies. 



Somites: Segments, or divisions of the body shown either by 

 constrictions on the surface, by appendages, or by internal 

 structure. 



Spicules: Bodies of various shapes, sometimes needle-like, again 

 three, four, or six pointed, which form part of the skeleton 

 of sponges. 



Spiracle: One of a number of openings along either side of the 

 body through which air is introduced into trachea. Best 

 developed in insects. 



Spiral valve: A spiral fold which occupies the interior of the intes- 

 tine of some fishes. 



Spongin: The silklike fibers that make up the skeleton of the 

 bath sponge. 



