DEVELOPMENT. 47 



and thick appendage usually bent in a sigmoid form, and can be 

 bent back under the mantle and thus be hidden. 



The penis is either hollow, in. which case the vas deferens pro- 

 ceeds to it as a closed canal passing through it to its extremity, 

 where it opens upon a small papilla as in Buccinum, or it opens 

 simpl}- as in Littorina, Oliva, Onchidiopsis ; or, in other cases it 

 is a solid body upon which the vas deferens passes along in the 

 form of a ciliated furrow continued upon it as a deep groove to 

 its extremity, as in Triton, Dolium, Cassis, Harpa. Yoluta, 

 Terebra, Strombus, Cypraea, etc. This last and most common 

 form of penis presents many varieties j in Cassis, for example, it 

 is pointed anteriorly, in Dolium it is enlarged anteriorly, in 

 certain species of Strombus it has a small appendage upon the 

 posterior side, and in Natica it presents at the end a whip-like 

 (flagelliform). in Dolium a claw-like appendage. Usually there are 

 large sack-like glands, which are placed on large pointed papillae 

 near the base of the penis ; they appear therefore as a row of 

 tubercles or processes, as in Littorina, Cassis, and Terebra, these 

 glands are placed upon special finger-like outgrowths of the 

 penis. 



Development. 



The eggs come in contact with the spermatozoa and are ferti- 

 lized in the oviduct or at the commencement of the uterus. The 

 eggs consist of a dark granular yolk; a germinal vesicle and one 

 or more germinative dots, enveloped by a thin vitelline mem- 

 brane. How the zoosperms penetrate this membrane is unknown ; 

 but they are introduced into the female tract by an act of copula- 

 tion in the bulk of the spiral prosobranchs, which possess a penis : 

 in the Trochoidea, Scuti branch s and C3^clobranchs, however, the 

 copulatory organ is wanting, and probably the spermatozoa dis- 

 charged into the surrounding water by the male, are thence taken 

 into the uterus. Of course the attached genera like Yermetus 

 and Siliquaria, and including also Magilns and Rhizochilus in 

 the Purpurinse cannot possibly fertilize in any other way. 



Yery few prosobranchiates are viviparous. The eggs are 

 usually enclosed, a number together, in tough leathery capsules, 

 within which they undergo their larval stage of development. 

 These capsules are variously aggregated, according to the genera. 



