THE PULMONATA. 



443 



vas deferens, which may be dilated in part of its course into 

 a veslcula seminalis. 



Fig. 123.— I. Peronia vernm/Ma.—a, atins ; pi, pnlnionary aperture; (/, genital aper- 

 ture ; fs, seminal a;roove; p. openinG: for the penis. 



U. Genc-i-ative ortians of the same animal, the ovotestis beins omitted.— j7a?, gland 

 wliicli famishes a glairy secreti(m ; od, oviduct ; vd, vas deferens; i, intestine ; a, 

 anus; rs. receptaenlum s^eminis ; p, aperture of the penis; p'. penis: es, seminal 

 duct ; ap, glandular appendage ; m, retractor muscle of the penis. (After Kefer- 

 stein.) 



IIT. Blind end of a follicle of the o\o\qM\9. f^^ TJelir pomat'w . At the apex the sperma- 

 tozoa are seen in different sta>j:es of development. Ihe fully-formed spermatozoa 

 floatintr in bundles in the cavitv of the follicle. Lower down, ova are developing 

 in the walls of the follicle. (After Keferstein and Ehlers.) 



The ova are impreprnated hi<rh up in the oviduct, and are 

 invested by a relatively very large mass of albumen and in- 

 closed within a thick, sometimes calcified, chorion. The mass 

 inclosed by the latter may be a tenth of an inch or more in 

 diameter, while the proper ovum may have not more than a 

 twelfth of that size. 



There is no trustworthy evidence of the existence of tlie 

 opisthobranchiate Gasteropods before the epoch of the Trias, 

 but it is to be remembered that the great majority of these 

 animals have no shells. Of the rest of the preceding groups 

 of Odojifophora^ representatives are known as far bnck as 

 the middle of the Palaeozoic epoch, while Pteropoda^ Hetero- 



