‘120 MOLLUSCA. 
the surrounding fluid in which they float about, having their 
motions in a great measure regulated by its changes. It is, 
however, probable, that, by means of some contractile 
movements, they are capable of varying their density, and 
of rising or sinking in the water. They swim slowly, even 
with their utmost efforts. The animals of this section be- 
long to the classes which Cuvier has termed Cephalopoda 
and Pteropoda. 
Crass ]—CEPHALOPODA. 
’ Fins in the-form of-tentacula, surrounding the mouth. 
The Cephalopoda, in reference to their external appear- 
ance, may be regarded as consisting of two parts ; the tunic 
xr sac, which contains the viscera, and the head, surround- 
od by the tentacula. The skin is usually mottled with 
ninute coloured spots, the colour varying in intensity in differ- 
ent spots, and evenin different parts of the samespots. These 
are confined to a thin layer on the outer surface of the true 
skin. In a living state these spots change their colour in 
rapid succession as if a coloured fluid was expelled from 
chem or replaced in variable quantity. The sac is, in some 
species, in the form of a purse, destitute of any appendages, 
while in others, it exhibits fin-like expansions. It varies: 
considerably in its consistence : in some, it is strengthened 
on the back internally, by corneous ribs or testaceous plates, 
and in others, it is protected externally by spiral shells. In 
some species, it is connected with the head by an interven- 
ing space, which may be regarded as a neck, but in others, 
the tunic and head are continuous behind. In all, it ex- 
hibits, after death, great changes of colour. 
On the summit of the head there is a flattened disc, in 
