THE MOLLUSCA 115 



the largest known species (Tridacna gigas) sometimes weighs over 

 500 Ib. These huge molluscs are found in the lagoons, among 

 coral reefs. The animals themselves are wonderfully coloured, 

 and gleam with iridescent tints. 



The eggs of molluscs vary greatly in number. The Oyster 

 (Ostrea ediilis) produces from 300,000 to 60,000,000 ; a Common 

 Snail (Helix aspersa) from about 40 to 100. The marine Gastropods 

 belonging to carnivorous families deposit their eggs in little leathery 

 cases of different shapes and sizes, each case containing several 

 eggs. The large clusters of eggs formed by the Whelk are well 

 known objects on the seashore, and the delicate cases containing 

 the eggs of Purpura (the Dog Whelk) may be seen in the crevices 

 of rocks or wooden breakwaters, each one about the size of a grain 

 of rice and fashioned like a wine-glass on a slender stem, the colour 

 being a delicate pink. 



The curious curled cakes of sand we sometimes find on the 

 seashore contain the eggs of the Natica, while the Doris disposes 

 her eggs in a long, gelatinous ribbon wound round and round 

 into a kind of rosette. Few molluscs take any care of their 

 eggs, but the " Violet Snail " (lanthinida) constructs a gelatinous 

 raft, which is filled with air-bubbles to act as a float ; to the under- 

 side it attaches its tiny eggs, and this egg-raft is fastened to the 

 foot of the little mollusc. 



