TETRABRANCHIATE CEPHALOPODS. 279 



chambers, and termed the " pericardium " (Owen). The re- 

 spiratory organs are in the form of four pyramidal branchiae, 

 two on each side. 



The chief masses of the nervous system are the cerebral and 

 infra-cesophageal ganglia, which are partially protected by a 

 cartilaginous plate, which is to be regarded as a rudimentary 

 cranium, and which sends out processes for the attachment of 

 muscles. The organs of sense are two large eyes, attached by 

 short stalks to the sides of the head, and two hollow plicated 

 sub-ocular processes, believed to be olfactory in their function. 



The reproductive organs of the female consist of an ovary, 

 oviduct, and accessory nidamental gland. 



There is no ink-bag, and the funnel does not form a com- 

 plete tube, but consists of two muscular lobes, which are simply 

 in apposition. It is the organ by which swimming is effected, 

 the animal being propelled through the water by means of the 

 reaction produced by the successive jets emitted from the 

 funnel. The function of the chambers of the shell appears to 

 be that of reducing the specific gravity of the animal to near 

 that of the surrounding water, since they are probably filled 

 with some gas secreted by the animal, or with water itself. The 

 function of the siphuncle is unknown, except in so far as it 

 doubtless serves to maintain the vitality of the shell. 



SHELL OF THE TETRABRANCHIATA. The shells of all the 

 Tetrabranchiata agree in the following points : 



1. The shell is external. 



2. The shell is divided into a series of chambers by plates 

 or " septa," the edges of which, where they appear on the shell, 

 are termed the "sutures." 



3. The outermost chamber of the shell is the largest, and is 

 the one inhabited by the animal. 



4. The various chambers of the shell are united by a tube, 

 termed the " siphuncle." 



Agreeing in all these fundamental points of structure, two 

 very distinct types of shell may be distinguished as charac- 

 teristic of the two families NautUictek*$&& Ammonitidce, into 

 which the order Tetrabranchiata is divided. 



In the family Nautilidce (fig. 249), the " septa " of the shell 

 are simple, curved, or slightly lobed ; the " sutures " are more 

 or less completely plain ; and the " siphuncle " is central, sub- 

 central, or internal (i.e., on the concave side of the curved shells). 



In the family Ammonitidtz (fig. 249), on the other hand, the 

 septa are folded and complex ; the sutures are angulated, zig- 

 zag, lobed, or foliaceous ; and the siphuncle is external (i.e., 

 on the convex side of the curved shells). 



