THE GASTROPODA 129 
not self-sufficient, and the union of two individuals is necessary for 
fertilisation. There are, however, instances of Pulmonates which 
have been insolated from the time of birth and have nevertheless 
laid eggs which have developed (Zonites cellarius, Limnaea). In all 
species possessing a penis, fertilisation is effected by copulation ; 
in species without a penis—e.g. Patella—artificial fertilisation 
is possible. During the venereal act the penis is thrust into 
the bursa copulatrix, when this latter structure is present, and 
discharges into it a quantity of sperm which subsequently fertilises 
the ova during their passage down the oviduct. Copulation and 
the act of oviposition that follows it take place at various seasons. 
In temperate regions they continue from early spring onwards, 
extending even into the winter, as is the case in Patella in the 
Fic. 107. 
Fic. 106. : +54 é 
Two Limnaea stagnalis in copulation, 
Spermatophore of Nanina the left one acting as male. I, tentacle 
wallacet, magnified. (After and eye ; II, penis; III, foot ; 1V, buccal 
Pfeffer.) veil. (After Stiebel.) 
Atlantic and Trochus striatus in the Mediterranean. In the 
stylommatophorous Pulmonates with a common genital orifice, the 
two copulating individuals mutually fertilise one another, each 
one acting as male and female, and the same is the case in the 
majority of Nudibranchs. In the hermaphrodites with distant 
genital apertures the same animal may act as male and female, 
but as a rule not simultaneously, unless, indeed, it unites with 
two or more individuals to form’ a chain, as may be seen in 
Limnaea, the Aplysiomorpha, ete. The copulation of two 
individuals is effected in the same manner as in the dioecious 
Gastropoda (Fig. 107). 
Ill. Empryovoey. 
The eggs may be laid or may develop within the maternal 
organism. In the oviparous species that do not copulate, the 
9 
