51 
Class II. Heteropoda. 
This Class includes a few small species which 
inhabit the open sea. The foot is laterally com- 
pressed, and resembles a keel; it is very movable, 
and serves as a rudder. Behind the keel, we find 
a sucker which the animal uses to attach itself to 
seaweed, and other floating objects. In front of the 
keel is the head, which projects like a muzzle, and 
is provided with strong teeth. The intestines are 
rolled into a coil. The sexes are separate. Some 
species are entirely destitute of a shell, and are quite 
transparent, so that the darker intestinal portion is 
all that is seen of the animal as it moves through 
the water; and others have a thin shell into which 
they can retreat entirely, closing it with a lid attached 
| to the tail-like extremity of the animal. 
Plate XXI. fig. u. Atanta Perontt is a small 
species which swims in shoals in the open Mediter- 
ranean Sea. 

Class III. Pteropoda. 
These are small animals found on the open 
sea, which are provided with two large wing-shaped 
fins. They are hermaphrodite; the head is not dis- 
tinctly defined, and they have no well-developed 
organs of vision; but the mouth is furnished with 
jaws and teeth, and is sometimes surrounded with 
tentacle-bearing suckers. Some are naked, and others 
provided with a shell, but they are all able to sink 
| quickly to the bottom of the water by drawing in 
their fins. We have figured Co borealis (Plate XX. 
fig. f) a common species in the Northern Seas, which 
| largely contributes to form the food of the Green- 
land Whale. 

Class IV. Gastropoda. 
These are much more familiar animals than 
the Fteropoda. A snail is at once recognised as 
possessing tentacles with eyes, a broad foot on which 
it creeps, and a shell. In the Slugs the shell is 
wanting. We may add that the shape and size of 
the foot is very variable, and that most of the 
species have a well-defined head. The form of the 
tongue and the different arrangement of the teeth 
are great aids to their classification. The larger 
divisions of Gastropoda are characterised by the 
Order I. 
structure and position of the organs of respiration, 
which are either gills or lungs. In some, the sexes 
are separate, and others are hermaphrodite. Only 
a few species are viviparous. Most of them lay eggs; 
either laying a few large ones surrounded by a shell, or 
depositing spawn in gall-like clusters and strings. The 
majority are marine; but many are found on land, 
or in fresh water. Their food is very various; some 
feed on plants, others on dead animal matters, and 

others again are carnivorous. 
Branchiata. 
Plate XX (right side), 
This Order is divided into two sections ac- 
cording to the position of the gills, and other cha- 
racters. In one small group the gills open behind 
the heart. These animals are hermaphrodite, and 
usually shell-less. There are sometimes tufted appen- 
dages on the skin. We have figured two species of 
this section. 
Fig. g. Holidia cwrulescens is remarkable for 
its delicate colour. 
Fig. h. Aplysia depilans is called the Sea Hare, 
from the shape of the second pair of tentacles. The 
eyes are placed between the two pairs of feelers. 
The animal is common on the southern and western 
coasts of Europe, and discharges a dark violet fluid, 
which was regarded by the ancients as a poison. 
The greater part of the Lranuchiata belong to 
the second section, in which the gills are placed 
before the heart. They are all provided with a shell, 
and the sexes are separate. The eggs are often laid 
in capsules, which are then fixed to foreign substances. 
The beautifully-coloured shells are much admired by 
collectors of curiosities, and large prices have often 
been paid for a single shell. It is perhaps only natural 
that more attention should have been paid to the 
form and colour of the shells, than to the anatomi- 
cal structure of the animals which inhabited them. 
The shell is a secretion from the outer integument 
of the body, the mantle, and consists of carbonate 
of lime. The opening of the shell is called the 
mouth and its circumference the lip, while its coil 
is called the whorl. Many water-snails have a horny 
or chalky lid on the back of the foot, with which 
they can completely close the shell; and land-snails 
close their shells in a similar manner in winter. 
We have given two illustrations of the group 
in which the leaf-like gills are arranged in a circle. 
Fig. i. Patella granatina, one of the Limpets, 
is characterised by a smooth plate-like shell, which 
clings with wonderful firmness to rocks and stones. 
The eyes are placed at the base of the two pointed 
tentacles, and the gill-plume is spiral and very long. 
Fig. k. Chiton squamosus resembles a wood- 
louse in form, and as the shell consists of 8 jointed 
plates, it can roll itself into a ball like some of these 
animals. They resemble the Limpets in their habits, 
and cling to rocks in the same way. 
Many species exhibit comb-shaped gills. 
Fig. 1. Haliotis Iris, the Ear-shell, derives its 
name from its shape; the inside is beautifully irides- 
cent. This shell is pierced with a number of holes, 
through which the animal protrudes thread-like appen- 

