CEPHALOPODA. 3 
guard against the resistance caused by the rapid motion through the water of a mass 
so large as the cephalic portion of the animal. To obviate this defect, a peculiar 
apparatus is found in various Cephalopods, which, capable of bemg instantly brought 
into action, provides an additional and firm attachment of the head to the body. 
This apparatus is variable in form, and, except in three genera in which it is 
not found, forms one of the most certain generic characters. It consists of one or 
more cartilaginous or fleshy protuberances, placed on each side either of the inner 
surface of the body or of the base of the head, which fit ito corresponding holes or 
depressions formed for their reception in the opposite part of the head or body. This 
apparatus, termed by M. d’Orbigny the apparatus of resistance (l'appareil de resistance), 
has relation to the swimming power of the animal, and is more or less complicated as 
that increases or diminishes. 
The respiratory apparatus consists of two or four lamelliferous branchieze or gills, 
lodged in chambers contained in the visceral sac, but separated from the viscera by a 
membranous partition. The number of these gills has been adopted by Professor Owen 
as an ordinal distinction ; and, in the system of classification proposed by him, to 
which I shall hereafter refer, the Cephalopods are divided into dibranchiate and tetra- 
branchiate orders according to the fact of their possessing two or four branchie. Into 
the chambers containing the gills, the water is freely admitted by a valvular aperture, 
and having served the purpose of respiration, flows, or is forcibly ejected by the 
muscular contraction of the body, through the excretory tube or funnel (¢”/undibulum). 
The water thus expelled in streams more or less powerful and frequently repeated, at 
the will of the animal, causes a retrogressive movement, which forms its principal 
mode of locomotion, from which circumstance the tube itself is called by M. d’Orbigny 
the locomotive tube. The body thus becomes the most important locomotive agent ; 
and as its size and shape must materially influence the retrogressive motion, we can 
readily conceive that they will have relation to the exigencies of the animal for 
swimming. Thus the pelagic species, in which the body, from its comparative size, 
and its cylindrical form and tapering extremity, is adapted to contain a large quantity of 
water, and to move through the sea with facility, are, as their necessities would require, 
pre-eminently powerful swimmers; while, on the other hand, in the littoral species, to 
which great retrogressive power would be not only unnecessary, but a source of 
frequent injury, the body is small and rounded, or depressed, so as to afford a broad 
surface on which the animal can rest upon the ground. 
Among the dibranchiate Cephalopods the circulation is performed by the agency of 
a central or systemic heart, of two lateral hearts, subservient to the propulsion of the 
blood through the branchie, and thence called the dranchial hearts, and of a venous 
system consisting of two principal vessels, vere cave, contained in a cavity called by 
Professor Owen the pericardium, and communicating freely with the branchial 
chambers, and of other subordinate trunks or vessels. In this cavity terminates the 
