430 MOLLUSCA. 



to the mantle ganglion (ganglion stellatum), which are in some 

 forms connected by a commissure (Fig. 345, Z) dorsal to the oeso- 

 phagus, and from the visceral portion two visceral nerves, fused 

 at their origin, to the viscera (Fig. 345, n). The visceral nerves 

 join behind in a ganglion (g), which gives off a right and left 

 nerve to the two brachial ganglia (p). The infra-buccal ganglion 

 gives off a nerve backwards (Fig. 345, k, and Fig. 342, XI), which 

 runs along the oesophagus and ends in a large ganglion on the 

 stomach (j). This ganglion givts off a nerve which anastomoses 

 with the visceral nerves (i). 



Organs of sense. Tactile sensibility is specially localized in the 

 arms of Dibranchs and tentacles of Nautilus. There is an organ 

 which is supposed to be olfactory, just ventral to the eye; it generally 

 has the form of a pit (Sepia), but it may be tubercular. It is 

 innervated from the cerebral ganglion. Osphradia are absent in 

 Dibranchs, but are supposed to be present in Nautilus (see below). 

 The otocysts in Nautilus are placed high up on the anterior ring close 

 to the cerebral ganglion (Fig. 342, XI), and are sometimes described as 

 being adjacent to the pedal centres ; they contain numerous otoconia. 

 In Dibranchs they are embedded in the floor of the skull, and they 

 form a kind of labyrinth, their walls being drawn out into short 

 diverticula; they each contain one large otolith, and are innervated 

 from the pedal ganglion by a nerve which arises from the cerebral. 

 The tube which connects them with the exterior in the embryo 

 persists as a caecal process from the otocyst. Damage to the otocysts 

 of Cephalopoda has been found to interfere with their power of 

 maintaining equilibrium. 



The eyes in most forms are extremely complicated in structure. 

 In Nautilus, however, they are very simple, being altogether without 

 refractive media, and consisting merely of a vesicle with an extremely 

 narrow opening to the exterior. The lining of the vesicle constitutes 

 the retina, and is continuous through the aperture with the external 

 ectoderm ; it consists of two layers separated by a layer of pigment. 

 The eye of Nautilus is therefore constructed on the principle of the 

 pin-hole camera, there being a dark chamber lined by the sensitive 

 membrane, and a minute hole for the entrance of light. 



In the Dibranchs (Fig. 346) the optic vesicle is closed, and its 

 front wall secretes a cuticular biconvex lens (L) ; part of this lens is 

 thrown down by the lining epithelium of the outer wall of the vesicle, 

 and part of it by the outer ectoderm. The lens is therefore theoreti- 

 cally in two parts, and is traversed by the front wall of the vesicle. 



