GLOSSARY. 293 



Segmentation or yolJc-division. One of the changes occurring in 



the egg after fecundation, by which it becomes divided into 



cells. 

 Segments or somites. The transverse rings which go to make up 



the body of an Arthropod or of an Annelid. Metamere is used 



in a more special sense. 



Septum. A partition bony, muscular, membranous. 

 Sesamoid bones. Small bones developed in the tendons. 

 Siagonopoda. The two pairs of maxillae and first pair of maxilli- 



pedes in certain Crustacea (Spence Bate}. 

 Siphon. The respiratory tube of certain bivalve Mollusca ; but 



it is also used in a more general sense. 

 Siphunculi. The hollow anal processes of the Aphides. 

 Somatic cavity. See C&loma. 

 Somites. See 



Species. Individuals having certain characters in common while 



absent in other individuals. 

 Spermatoa. " The nucleated cell in which the spermatozoa are 



developed" (Owen). 

 Spermatozoa. The minute moving flagellate plastides forming 



the male element. " The nature of the influence of the male 



element upon the female is wholly unknown " (Huxley} . 

 Sph&ridia. Minute transparent bodies found in the Echinoidea. 

 Sphenoid. A bone placed at the base of the skull, articulated as 



well with the bones of the face. 

 Spiculum amoris. The love-dart of the snail. 

 Spiracles. See Stigmata. 



Sporosac. "A gonophore destitute of obvious umbrella" (All- 

 man). 

 Statoblast. A gemmule enclosed in a peculiar bivalve shell in 



some of the Polyzoa, agamogenetically developed. 

 Steatopygons. When there is an unusual development of adipose 



matter posteriorly. 

 Stemmata. See Ocelli. 

 Stigma. A small opaque spot on the fore wing of certain 



insects. 

 Stigmata. Pores through which air is admitted into the tracheal 



vessels of insects. 

 Stipes. A stalk ; the basal portion of the maxillae of insects. 



