MOLLUSKS 



180 



Fig. 98: NOR'I'HEKN SAND- 



C'llLLAU SNAIL. Long 



Island Sound. 



are often not only tactile organs but serve as eye-stalks. In the 

 land-snails and slugs the little cup-like eyes are at the extremity 

 of the feelers while in sea-snails they He 

 at the hase of the tentacles, or only half 

 way up. The eggs of snails are sur- 

 rounded by gelatinous envelopes or en- 

 closed in parchment-like cocoons of 

 definite shape. Some of the eggs of 

 land-snails contain a great deal of nutri- 

 ent jelly and may be covered with a 

 firm, smooth shell. Indeed, they may 

 be as large as the egg of a pigeon as in 

 the case of our American land-snail, 

 BiiUiiiii.<!. The early stages of segmen- 

 tation in the developing eggs of snails 



are quite similar to those of the flat-worms, and this proliably 

 indicates that both flat-worms, and mollusks are descended from 

 a common stock. In many of the sea-snails the larva becomes a 

 free-swimming, pear-shaped creature propelled by one or more rings 

 of cilia around the place of its greatest girth, and having a bristle 

 of cilia at its blunt anterior end, and wdiat is most interesting, 

 the clams, mussels and ringed-worms (Annelids) go through a very 

 similar stage in their development. Later a pair of large flat lobes 

 grow out on either side of the mouth, and the edges of these lobes 

 are fringed with powerful cilia which enable the little mollusk to 

 swim rapidly through the water. The larva is now called a relirjcr. 

 Finally the veliger lobes degenerate, and the shell becomes so large 

 that the larva sinks to the bottom as a small snail. 



In slugs and some land and fresh-water snails the primitive 

 shell and operculum are sometimes cast off, and another shell which 

 remains throughout life maj' develop. This casting off of the shell 

 takes place before the little snail hatches from tlie egg. 



A gooil account of the development of snails is given by Kor- 

 sclielt and Heider in their "Textbook of Embryology," Vol. IV, 

 Macmillan, 1900. 



The Sand-Collar Snails, (Liniatia Iteros and Ncverita diiplicdtd, 

 Figs. 98-100). These large snails are found in shallow water 

 along sandy beaches, and are very abundant off the coasts of Long 



