﻿BRITISH FOSSIL CEPHALOPODA. 11 



tlon' of these oblique muscles, by enlarging the angle between them, enables the 

 shell to act as a fulcrum without attachment. This would tally well with the 

 absence of any impressed muscle-marks in either the recent or fossil shells, and with 

 the exigencies of the animal in his periodical moves. The next set of muscles are 

 those of the funnel : the main ones commence in the sides, at the junction of the 

 mantle and the hood, and increase by additional fibres coming from the dorsal pro- 

 cesses and the sides of the cartilage. They consist of an inner transverse set, which 

 pass across the shell-muscles, and an outer set, which become more longitudinal by 

 passing into the folds of the funnel. Another pair arise from the ventral horns of 

 the cartilage, and pass in smooth canals to the tip of the funnel, which they draw 

 away from the valve which lies within it. By the action of these muscles it is easily 

 seen how water can be driven out of the mantle cavity by the funnel itself, without 

 the motion of other parts of the body — a motion which would be inconvenient to an 

 animal fixed to a shell. The third important set of muscles are those connected with 

 the mouth, which arise from the under-surface of the cartilage : one pair of retractors 

 take their origin at the upper ends of the circumoesophageal processes, and pass 

 within the plates of the smaller mandible : another pair surround the cartilage out- 

 side the nervous collar, and pass partly into the labial processes, and partly into the 

 .lower mandible and to its odontophore. The protrusor is a muscle which passes 

 from the base of one posterior labial process and round the smaller mandible on its 

 upper side, thus acting at the same time as a depressor. 



The circulating system of the Nautilus has been found more difficult of elucida- 

 tion, but at the same time perhaps more interesting, than any other part of its 

 organisation. The heart is situated on the ventral side of the body, just above the 

 mantle cavity, and a little to the left. It consists of one chamber only, of quadrate 

 form (fig. 2, i), twice as broad as long, and which receives the branchial veins 

 (fig. 2, h) at the four corners, each entrance being guarded by a semilunar valve. 

 It is traversed within by fleshy columns, and gives origin to a less and a greater 

 aorta. The first arises from the under side near the centre, where its orifice is 

 guarded by two valves. It is ultimately divided into three branches ; one passing to 

 the nidamental gland in the female, a second to the intestinal mesentery, and the 

 third turns back to pass through an aperture in the fold of peritonaeum which has 

 been called the pericardium, to supply the siphon (fig. 2, F). The second or great 

 aorta (fig. 2, /) rises from the left hind corner, and has muscular parietes at first, 

 like an aortic bulb, beyond which it is guarded by a valve ; it first supplies the 

 gizzard and the generative organs, between which it passes backwards, it then turns 

 forwards again, sends a branch to the liver, runs along the dorsal side of the crop, 

 supplies the shell-muscles, and, finally, is distributed to the parts about the mouth 

 and the funnel. Besides these, in close connection with the heart, is an elongated 

 pyriform sac, which is closed nearest the heart, and, according to Prof. Owen, at 



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