OYSTERS 137 



increase in the oyster population of the seas. Taking 

 them all round, five million young oysters start life in 

 order that one may finally come to maturity, so many 

 and varied and incessant are the dangers, the predatory 

 enemies, the destructive effects of cold currents, bad 

 ground, and other chances of life and death on to which 

 the swarming swimming young are launched. 



The above brief history applies to the North Sea or 

 Channel oyster, which is also found (with other species) 

 in the Mediterranean. The American and the Portuguese 

 oyster differ from it in being of distinct sexes, and in the 

 fact that the eggs are discharged into the sea by the 

 females, and are there fertilised by the sperm discharged 

 by the male oysters, instead of in the parent's body. 



Other " molluscs," such as snails and whelks, enclose 

 their fertilised eggs, when they lay them, in egg-shells. 

 Some snails enclose a single egg in a shell which is 

 filled up with clear liquid corresponding to the " white " 

 of a bird's egg in which the egg floats and develops. 

 The eggs of the common snail are not bigger than a hemp- 

 seed, but some Indian snails lay eggs as big as those of 

 a robin, with a hard, calcareous shell, and the young 

 snail has quite a large coiled shell of its own before it 

 escapes from the egg-shell. So that it looks, when one 

 of the big snail's eggs is broken, as though a snail had 

 managed to get inside a bird's egg without making a 

 hole in it ! The whelks and their kind lay many eggs 

 in one shell or capsule, and the sea-slugs produce a sort 

 of firm jelly, in cords like vermicelli, the jelly enclosing 

 hundreds of little sacs filled with liquid, in which the true 

 germs or fertilised egg-cells float. These are all methods 

 for protecting the young in their earliest condition. One 

 of our pond-snails the Paludina keeps her eggs, whilst 

 they develop, inside the dilated end of the tube which leads 

 from the egg-producing organ or ovary to the exterior, 



