REPRODUCTION AND GROWTH 271 



Acera, Philine, Aplysia, and Nudibranchs generally. In the 

 great majority of shore molluscs the young are hatched as 

 free-swimming larvae, but a small number of forms are 

 viviparous (e.g. L. rudis and L. neritoides). 



Gasteropods. — The tendency to abbreviate the larval 

 period in Littorinas in relation to the position occupied on 

 the shore has already been noticed. The life-history of 

 three common species of Littorina has been worked out by 

 Tattersall {op. cit.) Jrom whose paper the following details are 

 taken. The eggs of Littorina littorea are pink in colour and 

 are enclosed, usually singly or in pairs, in small transparent 

 capsules shaped like a soldier's *' tin " hat, with the eggs 

 lying in the crown (see Fig. 18). The diameter of the capsules 

 varies from '6 mm. to '9 mm., that of the eggs themselves 

 from '15 mm. to •16 mm. Instead of adhering in masses 

 which are attached to weeds, etc., as in Nudibranchs, for 

 example, the capsules are separate and are laid freely on the 

 sea-shore, where they have been found in numbers by 

 Tattersall among the fine shore deposit at low-water. The 

 larva hatches as an early veliger and the pelagic life is 

 prolonged. The total number of eggs a single female may 

 deposit is estimated roughly at about 5000. Oviposition 

 may proceed intermittently for a month or more, the original 

 act of copulation sufficing for the whole of the eggs laid. 



As contrasted with that of L. littorea the life-history of 

 L. ohtusata presents the following characteristics : (i) the 

 egg capsules are aggregated in masses and attached to weeds ; 

 (2) the early veliger stage is suppressed and the egg hatches 

 as a fully formed veliger larva ; (3) the free-swimming larval 

 life is much abbreviated. As already pointed out, Littorina 

 neritoides is viviparous. 



The Trochidae deposit their eggs on the under side of 

 stones and on seaweed ; each ovum is contained in a separate 

 capsule and the capsules may be either all glued together into 

 an irregular mass of varying size or be deposited separately. 

 According to Robert (1901), agglomerated spawn is formed 

 by Tr. granulattis, Tr. striatus, Tr. conuloides, Tr. exasperattis, 

 while Tr. magus, Tr. cinerarius, Tr. crassm lay theireggs singly. 



