MOLLUSKS 139 
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 lie ; 
at the base 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, Fig. 98; NORTHERN SAND- 
COLLAR SNAIL. Long 
Island Sound. 
Bulimus. The early stages of segmen- 
tation in the developing eggs of snails 
are quite similar to those of the flat-worms, and this probably 
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 what 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 veliger. 
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 may develop. This casting off of the shell 
takes place before the little snail hatches from the egg. 
A good aecount of the development of snails is given by Kor- 
schelt and Heider in their ‘Textbook of Embryology,” Vol. IV, 
Macmillan, 1900. 
The Sand-Collar Snails, (Lunatia heros and Neverita duplicata, 
Figs. 98-100). These large snails are found in shallow water 
along sandy beaches, and are very abundant off the coasts of Long 
