14 THE PHYSIOLOGY OF REPRODUCTION 



eggs.'' On our own coasts Nudibranchs come to shore to lay 

 their eggs from January to April. Patella spawns from October 

 until the end of the year. Purfura hpillus is said to be most 

 active during the same season, but it breeds to some extent 

 throughout the year. Buccinum undatum breeds from October 

 until May, whereas Littorina breeds all the year round.^ 



Among the land-MoUusca there is a more marked periodicity 

 in the breeding season than among the marine forms. In 

 temperate chmates breeding is restricted to the summer. In 

 the tropics the occurrence of the breeding season is generally 

 determined by the alternations of wet and dry seasons. In 

 other cases, where there are no great seasonal changes, the 

 land-Mollusca may breed all the year round.^ The snails of 

 the Mediterranean area, according to Semper, arrive at sexual 

 maturity when they are six months old, and before they are 

 fully grown. Those individuals which reach this age in the 

 spring deposit eggs a second time after the heat of the summer 

 is over, and so experience two breeding seasons in the year, 

 with an interval of a few months between them during the 

 hot weather. Semper shows, further, that individuals of the 

 same genera, or perhaps even of the very same species, in the 

 damper and colder chmates of the north, do not lay eggs till 

 development is complete ; while in the dry, warm region of the 

 Mediterranean, they have produced two lots of eggs before they 

 are fully grown. This is because completion of growth and 

 sexual maturity do not necessarily coincide. In a similar way, 

 in the pond-snail {Limncea) the minimum of temperature which 

 admits of the assimilation of food, and so of growth, is much 

 above the winter temperature of egg-deposition. 



In tropical climates, where the variation in temperature 

 throughout the year is reduced to a minimum, the periodicity 

 in the breeding habits of animals is to a considerable extent 

 obliterated, at least in so far as it is dependent upon tempera- 



1 Lo Bianco, " Notizie biologische riguardanti speoialmente il periodo di 

 maturitk sessuale degli animali del golf o di Napoli." Mitth. Zool. Stat. Neapol, 

 vols. viii. and xiii. Much valuable information concerning the breeding 

 habits of MoUusca and other animals, inhabiting the Bay of Naples, is given 

 in these papers. 



' Cook, "MoUusca," Camh. Nat. Hist., vol. iii., London, 1895. 



' Semper, loc. eit. 



