66 DIBRANCHIATA.—SEPIADA, 
thoroughly understood. Milne-Edwards, whose 
opinion is entitled to the highest respect, gives the 
following explanation of it:—‘ The skin of these 
animals is furnished with a number of differently- 
coloured spots, which alternately appear and dis- 
appear, and if a portion is put under a microscope, 
it may be perceived that these changes depend on 
the contraction of small vesicles filled with a 
coloured liquid, which reach from the surface of 
the skin to a considerable depth. When one of 
these spots appears, the liquid, corresponding here 
to the pigment in the other case, is propelled 
towards the superficial part of the vesicle, and there 
displays itself; whilst during its disappearance it 
is forced into the deeper parts by the contraction 
of this superficial point itself, which then becomes 
almost invisible.’ * 
FAMILY SEPIADA. 
(Cuttles and Squids.) 
The lingering rudiment of a vertebrate skeleton 
in these animals has been already noticed ; their 
body encloses, however, a solid support of quite 
another nature, which represents the true shell 
so characteristic of Mollusca generally. Within 
the substance of the mantle, if we slit it up along 
the line of the back, we find an oblong cavity, 
within which lies loose, and unconnected with it a 
large plate, horny in some species and shelly in 
others. The pen of the common Squid (Loligo 
vulgaris) is of the former texture; the substance 
called cuttle-bone, so often found on sandy beaches, 
is of the latter; but both are strictly analogous to 
* Edinb. New Phil. Journ. XVII. 319. 
