62 CEPHALOPODA. 



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 b}'- 

 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 tliey are 

 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 Teteabranchiata. — The shells of all the 

 TdrabrancMata agree in the following points : — ■ 



1. The shell is external. 



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

 or " septa," the edges of whicli, where they appear on the 

 surface, 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 traversed 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 Nautilidcc and Ammonitidce, into 

 which the order Tetrahrancldata is divided. 



In the family Nautilidce (fig. 449), the "septa" of the 

 shell are simple, curved, or sligiitly lobed ; the " sutures " are 

 more or less com})letely plain ; and the " siphuncle " is cen- 

 tral, sub-central, or internal {ix., on the concave side of the 

 curved shells). 



In the family Ammonitidce (fig. 449), on the other hand, 

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

 zigzag, lobed, or foliaceous ; and the siphuncle is external 

 {i.e., on the convex side of the curved shells). 



In both these great Ujpcs of shell, a series of representative 



