284 MOLLUSCA.-—CLASS V. CEPHALOPODA. 
the deep waters of the ocean, and, like the nocturnal swimmers of the 
preceding class, are seldom to be seen excepting at night. They exhibit, 
like the Pteropoda, a great degree of sociability, swimming for the most 
part in herds, and with great activity of movement; are exceedingly 
voracious, swallowing fishes, pteropods, and everything that comes in 
their way ; and in their turn are destructively preyed upon by the supe- 
rior inhabitants of the deep. They are provided, however, with a curious 
means of defence in this emergency ; a contrivance which none but the 
Great Artificer could have devised. In the interior of the body is a 
cavity, in which a kind of inky fluid is generated of greater or less in- 
tenseness, and the animal has the faculty of discharging it when in a 
state of alarm, for the purpose of darkening the surrounding water, so as 
to be enabled tu escape the vigilance of pursuers by making off unseen 
in an oblique or opposite direction. This protective property is naturally 
more strongly developed in those Cephalopods which are destitute of a 
calcareous envelope*, and in those which wander defenceless upon the 
open sea, than in those of more solitary habits, which find refuge in cavi- 
ties of rocks. The same variety of adaptation is observable too in their 
locomotive powers ; the wandering Cephalopods are furnished with a pair 
of lateral fins to assist them in their passage through the element, some 
having the addition of a caudal fin to enable them to swim backwards, 
whilst others have their tentacles webbed around the base like the foot of 
a duck, serving not only to increase their power of swimming, but en- 
abling them to dart out of the water, after the manner of the flying-fish. 
The suckers of these animals have a very formidable character, and 
assist them greatly in their cruel and voracious habits ; sometimes they 
are pedunculated, assuming the appearance of claws, and the fishermen 
are greatly afraid of them. The sense of vision is very strong in the 
Cephalopods ; many of them have the power of turning their great eyes 
completely round like the chameleon, and the skin of these mollusks, as 
* The skilful anatomists of the Nautilus do not appear to have discovered any reservoir for 
the inky fluid in that well-protected Cephalopod, or we should have hazarded the conjecture 
that something of the kind exists, from the black coating which the shell always exhibits 
upon the involuted convexity of the spire. 
