THE PULMONATA. 



443 



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

 a vesicula seminalis. 



Fig. 123.— I. Peronia verrvculata.—a, anus : pi, p-ilmonary aperture; g, genital aper- 

 ture ; fs. seminal groove : p. opening for the penis. 



II. Generative orjrans of the same animal, the ovotestis beins: omitted.— gal, gland 

 which furnishes a glairy secretion ; od, oviduct : vd, vas deferens; i, intestine ; a, 

 anus; rs, receptacnlum seminis ; p, aperture of the penis; p' '. penis: cs, seminal 

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

 slein.) 



III. Blind end of a follicle of the ovotestis of Helix pomatia. At the apex the sperma- 

 tozoa are seen in different, stages of development, Ihe fully-formed spermatozoa 

 floating in bundles in the cavity of the follicle. Lower down, ova are developing 

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



The ova are impregnated high 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 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 Odontophora, representatives are known as far bick as 

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



