MOLLUSCA PROPER. 201 
wound obliquely round a central axis or pillar, increasing gra- 
dually in size to the mouth. The last whorl is the largest, and 
is termed the “ body-whorl.” The mouth of the shell in many 
forms is unbrokenly round or “entire” (fig. 104, a), and it is 
found that most of these shells subsist upon vegetable food, as, 
for instance, the common periwinkles. In others, again (fig. 104, 
6), the mouth of the shell is notched or is produced into a canal, 
as in the common whelk, and it is found that these live upon 
animal food, or are “carnivorous.” There may be more than 
one of these canals or tubes, but they do not necessarily indi- 
cate the nature of the food, as their function is to protect the 
respiratory siphons. 
The Gasteropoda are divided into a good many groups, of 
which the’ more important may be briefly noticed, the fore- 
going applying chiefly to the ordinary forms, which, therefore, 
need no further description, The remaining members of the 
water-breathing Gasteropods are divided into two sections, 
differing a good deal from the typical forms of the class in 
many respects, 
As examples of the first of these may be taken the sea-slugs 
and sea-lemons (Vudibranchiata), specimens of which may at 
any time be found creeping about on sea-weeds, or attached 
to the under surface of stones at low water. These slug-like 
animals (fig. 105) are wholly destitute of a shell when fully 
grown, but possess an embryonic 
shell when young. When there 
are any distinct respiratory or- 
gans, these are in the form of 
gills, placed, without any protec- 
tion, upon the back or sides of = 
the body. The head is furnished Fig. 105.— Nudibranchiata. Doris 
with tentacles, which do not ap- Yohnstont, one of the sea-lemons. 
pear to be used as organs of 
touch, but are more probably connected with the sense of 
smell ; and behind the tentacles are generally two eyes. The 
nervous system is extremely well developed, and would lead to 
the belief that the sea-slugs are amongst the highest of the 
Gasteropoda. Locomotion is effected, as in the true slugs, by 
creeping about on the flattened foot. 
The last remaining group of the “ branchiate”’ Gasteropods is 
that of the Heteropoda (fig. 106), comprising a number of curious 
