REPRODUCTION 89 
Clausiliide 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 (¢.g., Jorunna Johnstont, 
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 cygnea) has been com- 
puted to lay from 14,000 to 2,000,000; the Painter’s 
Mussel (Unio 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 
