CLASS V 



CEPHALOPODA 



587 



tory funnel divided ; ink-bag absent ; arms represented in existing Nautili by lobes 

 and numerous tentacles, which are tvithout hooks and suckers. Cambrian to 

 Recent. 



Fig. 1098. 



Nautilus pompilius Linn. Recent ; Indian Ocean. Shell with contained soft 

 parts seen from the left side, the .shell b^ing cut through along the median 

 line, a, Mantle ; h, Dorsal lobe of the mantle ; c, Hood ; d, Hyponome, or 



Our knowledge of the soft parts of the Tetrabranchiates is based entirely 

 upon the single existing genus Nautihis (Fig. 1098). The soft parts are con- 

 tained in the outer- 

 most compartment 

 (living chamber) of 

 the shell, the ventral 

 portions being on 

 the external side. 

 The body is short 

 and thick, and the 

 head separated from 

 the remaining por- 

 tion. Around the 

 mouth are about 

 ninety external fili- 

 form tentacles, 

 placed upon the 

 edges of lobes, and 

 their basal parts 

 when contracted are 

 lodged in fleshy 

 sockets or sheaths. 



rrii • f, , 1 " ambulatory funnel "; p, Nidamental gland ;/i, Muscle for attachment ; o, Eye ; 



± ne pair OI tentacles «, Siphuncle ; t, Tentacle.s ; x. Septal chamber (after R. Owen). 



on the inner or 



dorsal side are fused so as to form a thicker muscular lobe or hood, which 

 serves to close the aperture of the shell when the animal is withdrawn into 

 the living chamber. On the ventral side of the head and tentacles, but 

 separated from them, is a very thick muscular leaf, having the free edges 

 external and rolled in upon themselves (Fig. 1098, (/). This is the so-called 

 ambulatory funnel of authors generally (Jiyponome of Hyatt), and its cavity 

 is contracted anteriorly and dilated posteriorly, where it opens into the 

 branchial chamber. It serves to conduct water which is taken by suction 

 into, and then violently expelled from the gill cavity of the mantle, thus 

 driving the creature backward by the force of reaction. Kerr suggests that the 

 structure of the infolding edges of the hyponome and the muscular character 

 of this organ would enable the animal to unroll and flatten it out so as to be 

 available for crawling. It is supposed to be homologous with the foot of 

 Gastropods, and this suggestion, if true, would show that it had not entirely 

 lost its normal functions in primitive forms of Cephalopoda. 



On either side of the head, near the pair of lateral tentacles, is placed a 

 large eye of primitive structure, which is supported on a short peduncle. 

 The mouth is in the centre of the lobes and groups of tentacles, the tongue is 

 fleshy, and the radula armed with numerous rows of plates and hooks. The 

 remarkably powerful jaws (Figs. 1099, 1100) are largely composed of a dark 

 horny substance, only their points being calcified. Similar calcified beaks are 

 not uncommon in Mesozoic terranes, being found either associated with 



