GLOSSARY. 191 



Radial symmetry. With the parts symmetrically radiating from a com- 

 mon center. 



Radius. One of the parts of the jaw apparatus of an echinoid; from 

 center to periphery. 



Radula. The flexible membrane of an odontophore that bears the teeth. 



Ray. One of the arms of a starfish. 



Rectum. The posterior division of the alimentary canal. 



Renal organ. An organ that excretes nitrogenous wastes and other 

 materials. 



Reservoir. A place where anything is stored; the poison sac of a bee. 



Respiratory tree. The respiratory mechanism of some holothurians. 



Retractor muscle. A muscle that withdraws an organ or portion. 



Rostrum. The anterior spine of a lobster and of other crustaceans. 



Rotula. One of the calcareous pieces of the jaw of an echinoid. 



Rudimentary. When applied to adult animals means permanently unde- 

 veloped; vestigial. 



Sagittal. In or parallel to the mesial plane. 



Salivary gland. In invertebrates applied to any gland that opens into 



the mouth cavity. 

 Scaphognathite. The epipod of the second maxilla of certain Crustacea, 



that is used in baling water. 

 Scolex. Anterior portion of the tapeworm. 



Segment. One of a series of divisions of an animal's body or appendage. 

 Segmentation. Frequently applied to the cleavage of an embryo. 

 Seminal receptacle. A sac in which spermatozoa are stored. 

 Seminal vesicles. The sacs that inclose the testes of an earthworm. 

 Septmn. A plate that divides two spaces. See Nereis. 

 Sessile. Fixed; without the power of locomotion. 



Seta. A small spine of an annelid that is usually of service in locomotion. 

 Setigerous gland. A gland that forms setae. 

 Sexual. Of or pertaining to sex or sexes. 

 Shell gland. A gland that secretes the shell; sometimes applied to the 



kidneys of Entomostraca. 

 Siphon. Tubes for the transmission of water in Mollusca. 

 Somite. Metamere; one of the serial body-segments of an animal as an 



annelid. 

 Sperm. Spermatozoon; male reproductive cell. 

 Spermary. Male reproductive body. 

 Spermatheca. A seminal receptacle, used for storing spermatozoa in the 



female. 

 Spermatozoon. Male reproductive cell. 

 Sperm-sphere. A mass of spermatozoa in the earthworm. 



