uently brought immediately on the side of the head, and are very
large in proportion to the size of the animal. A skin corresponding
to the mantle envelops the body, and the gills are on either side of
it;--the stomach with its winding canal, the liver, and heart occupy the
centre of the body, as in the two other classes. This class includes all
the Cuttle-Fishes, Squids, and Nautili, and has a vast number of fossil
representatives. Many of these animals are destitute of any shell; and
where they have a shell, it is not coiled from right to left or from
left to right as in the spiral of the Gasteropoda, but from behind
forwards as in the Nautilus. These shells are usually divided into a
number of chambers,--the animal, as it grows, building a wall behind
it at regular intervals, and always occupying the external chamber,
retaining, however, a connection with his past home by a siphon that
runs through the whole succession of chambers. The readers of the
"Atlantic Monthly" cannot fail to remember the exquisite poem suggested
to the Autocrat of the Breakfast-Table by this singular feature in the
structure of the so-called Chambered Shells.

[Illustration: Common Squid, Loligo, cut transversely: _a_, foot or
siphon; _b_, gills; _c_, mantle; _d_, shell; _e_, heart; _f_, main
cavity, with intestines.]

Cuvier divided the Mollusks also into a larger number of classes than
are now admitted. He placed the Barnacles with them on account of their
shells; and it is only since an investigation of the germs born from
these animals has shown them to be Articulates that their true position
is understood. They give birth to little Shrimps that afterwards become
attached to the rocks and assume the shelly covering that has misled
naturalists about them. Brachiopods formed another of his classes;
but these differ from the other Bivalves only in having a net-work of
blood-vessels in the place of the free gills, and this is merely a
complication of structure, not a difference in the general mode
of execution, for their position and relation to the rest of the
organization are exactly the same in both. Pteropods constituted another
class in his division of the type of Mollusks; but these animals, again,
form only an order in the class of Gasteropoda, as Brachiopods form an
order in the class of Acephala.

In the third division of the Animal Kingdom, the Articulates, we have
again three classes: Worms, Crustacea, and Insects. The lowest of