506 



MOLLTJSCA 



SUB-KINGDOM VI 



and numerous tentacles, which are without hooks and suckers. 1 Cambrian to 

 Recent. 



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

 upon the single existing genus Nautilus (Fig. 1049). The animal is contained in 

 the outermost 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 portion. Around the mouth are about ninety 



external filiform 



* - --^ ^ajjJlFTmfi- ^ tentacles, placed 



upon the edges of 

 lobes, and their basal 

 parts when con- 

 tracted are lodged 

 in fleshy sockets or 

 sheaths. The pair 

 of tentacles 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 with- 

 drawn into the 



Recent ; Indian Ocean. Shell with contained living chamber. On 



animal cut through along the median line, a, Mantle; &, Dorsal lobe of the ,1 r.^^^r.1 c.iVla nf 



mantle ; c, Hood ; d, Hyponome, or "ambulatory funnel " ; c, Nidamental gland ; tne ventral S1QI 



h, Muscle for attachment ; o, Eye ; s, Siphuncle ; t, Tentacles ; x, Septal chamber ^he head and ten- 



tacles, but separated 



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

 rolled in upon themselves (Fig. 1049, d). This is the so-called ambulatory 

 funnel of authors generally (hyponome 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 back- 

 ward by the force of reaction. Kerr 2 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. 1050, 1051) are largely composed of a dark, 



1 Professor Verrill homologises the lobes of Nautilus with the arms of the Dibranchiata, and 

 the tentacles with the small lateral tentacles that fringe the arms in a number of deep-sea forms be- 

 longing to that sub-class. 



2 Kerr, J. G., Anatomy of Nautilus pompilius (Proc. Zool. Soc., London, pp. 664-686), 1895. 



FIG. 1040. 



Nautilus 



