(T.l'II.VLOPODA. 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 Ccphalopods, which, capable of being 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 cither of the inner 

 surface of the body or of the base of the head, which fit into 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'apparcil 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 lamellifcrous branchiae 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 arc divided into dibranc/iiate and tetra- 

 branchiate orders according to the fact of their possessing two or four branchiae. 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 {infundibulum). 

 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 branchiae, and thence called the branchial hearts, and of a venous 

 system consisting of two principal vessels, vena cava, 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 



