3i6 THE CEPHALOPODA 



bend lying near the bifurcation of the vena cava on the dorsal side 

 of the rectum : this commissure may be seen in Spirilla (Fig. 27 7, j), 

 Sepia, arid Eledone, and in Ommatostrephes it bears a large ganglion, 

 known as the ganglion of the vena cava. In Ommatostrephes, 

 Ekdone, etc., 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 



