316 THE CEPHALOPODA 
bend lying near the bifurcation of the vena cava on the dorsal side 
of the rectum: this commissure may be seen in Spirula (Fig. 277, J), 
Sepia, and Eledone, and in Ommatostrephes it bears a large ganglion, 
known as the ganglion of the vena cava. In Ommatostrephes, 
Eledone, ete., there is yet another secondary nerve-centre at the 
origin of the brachial nerve. 
The stomato-gastric system of the Dibranchia consists of a pair 
of conjoined ganglia situated below the oesophagus and immediately 
behind the buccal bulb (Fig. 282, XIII) ; these ganglia are united to 
the cerebrals (the anterior lobe in the Decapoda) by the intermediary 
of the labial commissure, as has been described above. They give 
off nerves to the alimentary canal, these nerves extending as far 
as the stomach, where they enter a large ganglion, an offshoot from 
which anastomoses with the visceral nerve. 
The structure of the nerve-centres of the Cephalopoda resembles 
that of other Molluscs; they consist of a thick and continuous 
superficial layer of nerve ganglion cells beneath which is a fibrillar 
reticulum formed by the terminations of the centripetal nerve fibres 
and the prolongations of the superficial ganglion cells. These 
fibrillar centres are united by fibrillar connectives—namely, the 
cerebro-brachial, the cerebro-pleural, the pleuro-pedal, the pleuro- 
visceral, and the pleuro-brachial—many of which are short and 
covered over by the continuous layer of superficial ganglion cells. 
The Cephalopoda are well provided with sensory organs, possess- 
ing, in addition to the tactile structures, rhinophores, statocysts, 
and well-developed eyes. The sense of touch is more particularly 
localised in the arms of the Dibranchia and the tentacles of the 
Tetrabranchia. 
In all the Cephalopoda there is an olfactory organ situated near 
and below the eye on each side of the head. In sundry Oigopsida, 
such as Cheiroteuthis, Ctenopteryx (Fig. 259, ol), it is a projection, some- 
times pedunculated as in Cheiroteuthis and Doratopsis ; in Nautilus 
it is a cavity hollowed out in a tubercle; more generally it is a 
simple fossa of greater or less depth, as is the case in Sepia and 
the majority of the Dibranchia. The epithelium of this organ 
contains numerous sensory cells, and the nerve supplying it arises 
from the superior frontal lobe of the cerebral ganglion. This nerve 
is at first bound up with and appears to branch off from the optic 
nerve near a little tubercle situated on the latter, but it receives no 
fibres from it. 
In Nautilus the ciliated and lamellar pre-ocular and post-ocular 
tentacles are apparently accessory olfactory organs; the ciliated 
interbranchial papilla of each side is placed on a sensorial area 
innervated by the fibres of the two branchiae (Fig. 280, n.olf). 
The post-anal papilla (Figs. 275, 276, x) is also ciliated, but is not 
supplied by any special nerve. In the Dibranchia the branchial 
