THE PULMOXATA. 



443 



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

 a vesicula seminalis. 



IH 



Fig. 12.3.— I. Peronia verrucnloia. — a. ann= : ;/. pulmonary aperture : ^, genital aper- 

 ture : f«. seminal groove : p. opening for The penis. 



II. Generative organs of the same animal, the ovotcstis beine omitted. — gal. eland 

 which famishes a glairy secretion : ofi. oviduct : rd. vas deferens : i. intestine : a. 

 anus: 7'.*. receptaculnm" seminis : p. aperture of the penis: p/ . penis: c-9. seminal 

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

 stein.l 



in. Blind end of a follicle of the ovotestis of ^^7?>;)(W2/7#?a. At the apex the sperma- 

 tozoa are seen in different stages of devplnpmeut. ihe fully-formed spermatozoa 

 floating 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 impreo;nated hiffh up in the oviduct, and are 

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

 closei 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 evid<mce of the existence of the 

 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 0<Tonfophora^ representatives are known as far back as 

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



