98 OKGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. 



obtains in the marine Polychpeta, in which the ova and spermatozoa 

 are developed from the epithelium of the body-cavity (mesoderm), and 

 dehisced into the body cavity. Usually, however, special glands, the 

 ovaries and testes, are developed, which perform no other function 

 than that of secreting ova and spermatozoa (Echinoderms). 



As a rule, however, there are found associated with the male and 

 female generative glands accessory structures and a more or less com- 

 plicated arrangement of ducts, which discharge definite functions in 

 connection with the development of the generative products subse- 

 quent to their separation from the glands, and ensure a suitable 

 meeting between the male and female elements (fig 90). The ovaries 

 are provided with ducts, the oviducts, which are not rarely derived 



a 



Jls 



FIG. 91, a. The female organs of Pulex (after Stein). Ov, ovarian tubes ; 22s, receptaculum 

 semiuis ; V, vagina ; Gl, accessory gland, b, The male generative organs of a water-bug 

 (Nepa) (after Stein). T, testis ; Vd, vasa defereutia ; Gl, accessory glands ; D, ductus ejacu- 

 latorius. 



from structures serving quite another purpose (segmental organs). 

 The oviducts, in their course, may receive glandular appendages of 

 various kinds which furnish yolk for the nourishment of the ovum, 

 or albumen to surround it, or material for the formation of a hard 

 egg-shell (chorion). These functions may be sometimes discharged 

 by the ovarian wall (Insects), so that the egg when it enters the 

 oviduct has taken up its accessory yolk and acquired its firm egg- 

 shell. Very often the ducts also discharge these various functions, 

 and are divided into corresponding regions ; they are often dilated 

 at part of their course to form a reservoir for the retention of the 



