THE GASTROPODA 



125 



males of Ampullaria and the Heteropoda also possess a vesicula 

 seminalis, and the penis frequently is furnished with well-marked 

 superficial glands (Littorinidae, Cassis, Terebra, and the Heteropoda). 

 In some Taenioglossa, such as Pahidina and Pteroceras, and in 

 several Stenoglossa, such as Murex, Nassa, Purpura, etc., there are 

 two kinds of spermatozoa, the one normal and filiform, the other 

 vermiform ; the function of the latter kind is not yet explained. 

 In Paludina, for example, these two kinds of spermatozoa exist in 

 equal quantities, but the filiform kind, with a single cilium, 

 originate from spermatids which have increased but little in size 

 during the growth period, and contain the normal quantity of 

 nuclear substance : these are the eupyrenic spermatozoa of Meves. 

 The vermiform spermatozoa, on the other hand, have about six 

 cilia apiece, originate from spermatids which have increased largely 



FIG. 102. 



Follicles of the hermaphrodite gonads of Euthyneura. A, of Helix; I), of Eolis. a, ova; 

 b, developing spermatozoa ; c, common efferent duct. (From Lankester, after Gegenbaur.) 



in size during the growth period, and contain only a small quantity 

 of nuclear substance : they are known as oligopyrenic spermatozoa. 

 In the monoecious Gastropods the gonad ordinarily occupies 

 the same position and has the same relations as in the dioecious 

 Streptoneura, but it may be much more subdivided, especially in 

 certain Nudibranchs, viz. Phyllirhoe (Fig. 161, y) and Elysiomorpha. 

 It has always a duct with its proper external orifice and a penis 

 which is invaginable in most Euthyneura, but this latter organ is 

 absent in hermaphrodite parasitic Streptoneura. The gonad differs 

 from that of the dioecious Streptoneura in producing ova and 

 spermatozoa in the same individual. In the most simple arrange- 

 ment the two kinds of genital products are developed side by side, 

 as may be seen in Valmta and in the majority of the Tectibranchia 

 and Pulmoriata (Fig. 102, A). In the more specialised condition 

 there are male and female acini, the latter opening into the 

 spermatogenous sacs in Oncidiopsis, the Pleurobranchidae, .the 

 majority of the Nudibranchia (Fig. 102, B), with the exception of 



