THE GASTROPODA 12 
ur 
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 Paludina and Pteroceras, and in 
several Stenoglossa, such as Murex, Nassu, Purpura, ete., 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 
Follicles of the hermaphrodite gonads of Euthyneura. 4, of Helix; B, of Holis. 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. Phyllirhoé (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 Valvata and in the majority of the Tectibranchia 
and Pulmonata (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, 5), with the exception of 
