118 SEXUAL ORGANS. 



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 

 (flagelliforni), 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 thei'efore as a 

 row of tubercles or processes, as in Littorina, Cassis, and 

 Terebra : these glands are placed upon special finger-like out- 

 growths of the penis. 



The copulatory act usuall}^ takes place in the spring ; but in 

 Littorina it occurs throughout the season, and the female has 

 sometimes both large and minute eggs in her uterus. 



The eggs come in contact with the spermatozoa and are 

 fertilized 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 

 membrane. The zoosperms penetrate this memljrane through an 

 opening termed the micropyle. They are introduced into the 

 female tract by an act of copulation in the bulk of the spiral 

 prosobranchs, which possess a penis ; in the Trochoidea, Scuti- 

 branchs and Cyclobranchs, however, the copulatory organ is 

 wanting, and probably the spermatozoa discharged into the sur- 

 rounding water by the male, are thence taken into the uterus. Of 

 course the attached genera like Yermetus and Siliquaria, and 

 including also Magilus and Rhizochilus in the Purpurinae, 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. 

 Littorina deposits its eggs in gelatinous masses, and the outer 

 portion of the albumen of each egg hardens into a sort of shell ; 

 but ordinarilj^ an egg-capsule is formed, and then the separate 

 ova do not possess shells, but the capsule encloses a mass of 

 albumen which is common to all the ova within it — sometimes 

 several hundred. In this albumen the larvae move about before 

 leaving the capsule for the outer world. 



The number of eggs deposited by the prosobranchiate mollusca 

 is relatively small when compared with the lamellibranchiates, 

 but enormous in comparison with the pulmonates. A nidimental 

 string of Pyrula canaliculata contained fifty capsules, each 

 enclosing about a hundred eggs, say five thousand eggs in all. 

 Li Buccinum the number is much greater. 



The capsules are variously shaped and aggregated, and were 

 formerl}'- mistaken for and described and figured as zoophytes. 

 It will assist us in our survey of their forms to present the 

 classification of these bodies which was proposed by the cele- 



