506 MOLLUSCA SUB-KINGDOM VI 
and numerous tentacles, which are without hooks and suckers —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 
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- 
Fre. 1049. drawn into the 
Nautilus pompilius, Linn. Recent; Indian Ocean. Shell with contained living chamber. On 
rane ood eh eanGao cree ee yaa Rawal ae tS the ventral aide of 
eestor tk: 0, Eye; s, Siphuncle; t, Tentacles ; x, Septal chamber the head and ten- 
ry 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? 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 ashort peduncle. The 
riba 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 Navtilus 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. 
* Kerr, J. G., Anatomy of Nautilus pompilius (Proc. Zool. Soc., London, pp. 664-686), 1895. 
ss 
pitta ea Laan 
