MOLLUSC A— CEPHALOPODA. 17 



name Tetrahranchiata implies, this animal possesses four gills, while 

 the other recent cephalopods possess only two, hence they are classed 

 as Dibranchiata. An internal '' ink-bag," secreting an inky fluid 

 (sepia), generally occurs in the Dibranchiates (squids, etc.), but 

 is absent in Nautilus. The arms of the Dibranchiates are eight or 

 ten in number. They are provided with suckers (acetabula) on 



Fig. 1230, Nautilus pompilius. Section of shell with animal in place. (Owen's 

 figure ; after Clarke, Min. Pal.) a, mantle; b, its dorsal fold ; e, nidamental gland; 

 g, shell muscle ; i, sipho ; k, hyponome ; n, hood ; o, exterior digitation ; /, tentacles; 

 J, eyes ; x, septa ; z, body chamber. 



their inner side, or with a double row of hooks. In forms with 

 ten arms (Sepioidea) , two are developed into very long tentacles 

 with hooks or suckers only at their thickened extremities. In 

 Nautilus the arms are represented by lobes, and there are in addi- 

 tion numerous (90) tentacles which are free from hooks or suckers 

 (Fig. 1230, p). 



The body of Nautilus is short and thick, and it is lodged in the 

 last or body chamber of the coiled shell in such a position that the 

 ventral or under side lies on the exterior of the coil. The inner 

 or dorsal pair of tentacles is fused into a thick muscular lobe or 

 hood (Fig. 1230, n), which acts as an operculum when the animal 



