564 LECTURE xxn. 



straighten the zig-zag turns into which the vas deferens is thrown in 

 its usual relaxed state. 



In the female the position of the testicle is occupied by the ovary, 

 while the vas deferens is represented by a thick, and glandular 

 oviduct. There is no rudiment of exciting organ (clitoris) : the ge- 

 nerative aperture is situated a little within the edge of the branchial 

 cavity, and is a simple hole leading to the oviduct.* 



In Murcx the penis is proportionally smaller ; and, instead of a 

 complete vas deferens, penetrating to its extremity, there is merely a 

 groove along its surface, along which the semen flows. In Vohita 

 the exterior groove only runs to the base of the penis, and in 

 Strombus the male organ is a mere tubercle. 



In the Pidmonalia opercidata the organs of both sexes are in every 

 respect similar to those of the Pectinibranchiate order. In Paludina 

 {Helix vivipara, Lin.) the penis is closely united to the right tentacle, 

 and, like it, is retractile. The ovarium, in the female, is shown in 

 Jig. 204. by its lighter colour, combined with the liver and intestine, 

 with which it is lodged within the upper whirls of the shell. The 

 oviduct (^) is susceptible of considerable dilatation at the breeding 

 season, the young being developed therein : it terminates just within 

 the branchial chamber. 



In the male Limpet the testis is situated beneath the liver ; its 

 duct passes forwards and terminates near the vent on the right side. 

 The ovarium and oviduct have the same disposition in the female. 

 The Haliotis differs merely in having the genital orifice on the left 

 side. The spermatozoa in these non-copulant Gastropods have a long 

 body with a still longer filamentary tail : in Paluduia the body has a 

 spiral form, as is the case, likewise, with the shorter body of the 

 spermatozoa of the Helix and Limax. In the sperm of Paludina 

 there have also been observed long cylindrical bodies, from one of 

 the extremities of which project many delicate filaments having lively 

 motions. The ovarian ova present a round and sometimes elliptical 

 form, and consist of a finely granular and variously coloured yolk, 

 containing the germinal vesicle and nucleus, and inclosed in a thin 

 smooth membrane. L'pon this is laid the secretion of the albumini- 

 parous gland, as the ova traverse that cavity in the Pidmonata ; and 

 they are coated and often connected together by analogous secretions 

 in other orders of Gastropods ; fecundation having previously taken 

 place in the oviduct or uterus. 



The ova of the marine Gastropods are enveloped, before exclusion, 

 in mucous capsules, prepared by a special gland situated near the 



* CCCXXI. 



