CHAPTER V 



REPRODUCTION — DEPOSITION OP EGGS — DEVELOPMENT OF 

 THE FERTILISED OVUM — DIFFERENCES OF SEX — DIOE- 

 CIOUS AND HERMAPHRODITE MOLLUSCA — DEVELOPMENT 

 OF FRESH-WATER BIVALVES 



Reproduction 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 young from the egg before expelling them. 

 Such ovoviviparous genera are Melmiia^ Paludina^ Balea^ and 

 Coeliaxis among land and fresh-water Mollusca, and Cymha and 

 many Littorina amongst marine. The young of Melania tuher- 

 culata^ 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 ovoviviparous, e.g. Patula Cooperi^ P. Hemphilli^ and P. 

 rupestris, Acanthinida harpa^ MicropJiysa vortex^ Pupa eylin- 

 draeea and museorum^ Clausilia ventricosa, Opeas dominicensis, 

 Rhytida inaequalis^ etc. All fresh-water Pelecypoda yet exam- 

 ined, except Dreissensia^ are ovoviviparous. 



The number of eggs varies greatly, being highest in the 

 Pelecypoda. In Ostrea edulis it has been estimated at from 

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

 in Unio pietorum 200,000. The eggs of Doris are reckoned at 

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

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

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



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