PHYLUM MOLLUSCA 



727 



conical head, bearing eyes and a system of tentacles, and a 

 rounded sac-like trunk. Both head and trunk are very slightly 

 compressed, the direction of the compression being, as in Sepia, 

 from the antero-dorsal towards the postero-ventral side, and 

 are almost completely bilaterally symmetrical, only a very 

 slight disturbance of the symmetry being discernible. The 

 mouth, situated at the free extremity, is provided with a pair of 



Jaws irtf 



cart 



rnant 



cterv 



rruis 



. (538. Nautilus pompilius, diagrammatic lateral view of a female specimen enclosed 

 in its shell, cart, cartilage ; cten. ctenidia ; hd. hood ; inj\ funnel ; jaics, jaws ; iMint. mantle ; 

 'tiiant'. dorsal mantle fold overlapping the coil of the shell ; mus. position of lateral mass of 

 muscle ; nid. nidameutal glands ; sept, first septum ; siph. siphuiicle. (After Keferstein.) 



relatively enormous, partly calcined jaws (Fig. 638). Surrounding 

 the mouth is a series of bilaterally arranged lobes which represent 

 the fore-foot of other Molluscs. These are beset with numerous 

 slender tentacles, each provided with an elongated tubular sheath, 

 in the interior of which the greater part of the tentacle in the 

 retracted condition lies enclosed, only a small portion protruding. 

 Minute ring-like markings on the tentacle are due to the 

 presence of circular bands of muscular fibres. There are no 



