MOLLUSCA I CEPHALOPODA. 309 



large crop, which in turn conducts to a gizzard, and the intes- 

 tine terminates at the base of the funnel. On each side of the 

 crop is a well-developed liver. 



The heart is contained in a large cavity, divided into several 

 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-oesophageal 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 subocular 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 most probably 

 filled with some gas secreted by the animal. 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 : 



T. 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 character- 

 istic of the two families Nautilida and Ammonitida, into which 

 the order Tetrabranchiata is divided. 



In the family Nautilida (fig. 112), the " septa" of the shell 

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

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



