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 cellaring, 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 



Fio. 107. 



Two Limnaea stagnalis in copulation, 



Spennatophore of Nanina the left one acting as male. I, tentacle 



wattacei, magnified. (After and eye ; II, penis ; III, foot ; IV, 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, etc. The copulation of two 

 individuals is effected in the same manner as in the dioecious 

 Gastropoda (Fig. 107). 



III. EMBRYOLOGY. 



The eggs may be laid or may develop within the maternal 

 organism. In the oviparous species that do not copulate, the 



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