CHAPTER V 



EEPRODUCTION DEPOSITION OF EGGS DEVELOPMENT OF THE 



FERTILISED OVUM DIFFERENCES OF SEX DIOECIOUS AND 



HERMAPHRODITE MOLLUSCA DEVELOPMENT OF FRESH-WATER 



BIVALVES. 



EEPRODUCTION in the Mollusca invariably takes place by means 

 of eggs^ which, after being developed in the ovary of the female, 

 are fertilised by the spermatozoa of the male. As a rule, the 

 eggs are ' laid,' and undergo their subsequent development apart 

 from the parent. This rule, however, has its exceptions, both 

 among univalve and bivalve Mollusca, a certain number of which 

 hatch their yoimg from the egg before expelling them. Such 

 ovoviviparous genera are Melania, Faludina, Balea, and Coeliaxis 

 among land and fresh -water Mollusca, and Cymba and many 

 Littorina amongst marine. The young of Melania tuberculata, 

 in Algeria, have been noticed to return, as if for shelter, to the 

 branchial cavity of the mother, some days after first quitting it. 

 Isolated species among Pulmonata are known to be ovovivi- 

 parous, e.g. Patula Cooperi, P. Hemphilli, and P. rupestris, 

 Acanthinula liarpa, Micro;p]iysa vortex, Pupa cylindracea and 

 viuscorum, Clausilia ventricosa, Opeas dominicensis, Rhytida in- 

 aequalis, etc. All fresh-water Pelecypoda yet examined, except 

 Dreissensia, are ovoviviparous. 



The number of eggs varies greatly, being highest in the 

 Pelecypoda. In Ostrea cdulis it has been estimated at from 

 300,000 to 60,000,000; in Anodonta from 14,000 to 20,000; 

 in Unio lyictoruni 200,000. The eggs of icon's are reckoned at 

 from 80,000 to 600,000, oi Loligo and Sepia at about 30,000 

 to 40,000. Pulmonata lay comparatively few eggs. Arion ater 

 has been observed to lay 477 in forty-eight days (p. 42). Kests 



