MOLLUSCA I CEPHALOPODA. 355 



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. Good authorities, how- 

 ever, believe that the chambers of the shell are filled with water. 

 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 



,0 0-Q,Q 



Fig. 149. Diagram to illustrate the position of the siphuncle and the form of the septa 

 in various Tetrabranchiate Cephalopoda. The upper row of figures represents 

 transverse sections of the shells, the lower row represents the edges of the septa. 

 a a A mmonite or Baculite ; bb Ceratite ; c c Goniatite ; d d Ciymenia ; e e Nau- 

 tilus or Orthoceras. 



very distinct types of shell may be distinguished as character- 

 istic of the two families Nautilida and Ammonitidce, into which 

 the order Tetrabranchiata is divided. 



