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. 



CLASS I. 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 

 or sac, which contains the viscera, and the head, surround- 

 ed by the tentacula. The skin is usually mottled with 

 minute coloured spots, the colour varying in in tensity in differ- 

 ent spots, and even in different parts of the same spots. 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 

 them 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 



