REPRODUCTION 89 



Clausiliidse the eggs are also sometimes extruded 

 before hatching, as happens in Balea (Plate XIII., 

 Fig. 18) and some species of Clausilia itself. 



The eggs are isolated in the Amphineura, the 

 more primitive Gastropoda, in the Scaphopoda, and 

 the Pelecypoda. In the majority of the aquatic 

 Gastropoda they are either enclosed in tough cap- 

 sules, which may be deposited apart (as in Purpura), 

 or in irregular clusters (as in the Whelk, Plate XXX., 

 Fig* 3)« Or the spawn consists of large numbers 

 of eggs agglutinated in gelatinous masses, or spread 

 out in the shape of a strap or ribbon, in which the 

 eggs are arranged in rows. This "nidamental rib- 

 bon" is sometimes coiled up like a watch spring and 

 attached by one of its edges (e.g., Jorunna Johnstoni, 

 a Sea Slug, Plate XXX., Fig. 4). The nidamental 

 capsules of the Cuttlefish are clustered like grapes, 

 each containing but a single embryo. In Ianthina, 

 the purple Sea Snail, the egg capsules are carried 

 closely packed on the under side of a raft or float, 

 formed by the parent, and attached by one end to 

 her foot (Plate XXX., Fig. 2). 



The number of eggs laid at a time varies in the 

 different species, but is greatest in the Pelecypoda. 



The Swan Mussel (Anodonta cygncea) has been com- 

 puted to lay from 14,000 to 2,000,000 ; the Painter's 

 Mussel (Unto pictorum) 220,000; the Common Oyster 

 from 600,000 to over 1,800,000; while a large American 

 Oyster may, it has been said, contain more than 



