18 THE MOLLUSC A 



epipodial tentacles of Rhipidoglossa, the pallial tentacles of Lamelli- 

 branchs, etc., which therefore have the more special function of 

 tactile organs. 



Taste organs in the form of sensory bulbs or cups have been 

 shown to exist in the buccal cavity or round the mouth of several 

 Molluscs. There are several kinds of organs of olfactory or some 

 analogous function ; such are the cephalic rhinophores and the 

 pallial osphradia. The rhinophores are situated on the head, and 

 are frequently borne on more or less prominent appendages 

 resembling tentacles (Fig. 163, /); or in other cases they have the 

 form of a pit, as in the Cephalopods (Fig. 259, ol). The nerve of 

 each rhinophore is supplied by the corresponding cerebral ganglion, 

 and is sometimes partially united to the opcic nerve. The osphradia 

 (Ray Lankester) are situated near the entrance to the pallial cavity 

 (Figs. 58 and 89, 0s; 99, XVI.), on the course of the branchial 

 nerve, but sometimes, as a result of specialisation, they are situated 

 on separate ganglia. The osphradia have the form of ridges or 

 pits Jined by sensory epithelium, and their function is to test the 

 respiratory fluid. It has been demonstrated, in certain Lamelli- 

 branchs at any rate, that the nerve-fibres suoplying the osphradia 

 originate in the cerebral ganglia. 



The otocysts (de Lacaze-Duthiers) are invaginations of the 

 integuments of the foot. In the Protobranchs (Pelseneer) and in 

 Mytilus (List) they are otocrypts, that is to say, they are still open 

 invaginations ; but in all other Molluscs these organs are closed 

 and contain auditory granules or otoliths suspended in a fluid 

 secreted by the wall of the otocyst, this latter structure being 

 furnished with sensory and ciliated cells. The otocysts, even when 

 they are attached to the pedal ganglia, are innervated from the 

 cerebral ganglia (Koren and Danielssen, de Lacaze-Duthiers, Leydig; 

 Figs. 123, C ; 146). They are absent both in the young and adult 

 Amphineura and in the adult stages of various sessile Molluscs. 

 It is through the agency of these organs that the animal is able 

 to perceive disturbances in the surrounding medium, and the re- 

 sistance offered to the locomotory apparatus : through them the 

 creeping Molluscs preserve their orientation and swimming Molluscs 

 their equilibrium. 



Tlie Eyes are normally cephalic structures, one pair in number, 

 symmetrically placed on or at the bases of the cephalic tentacles. 

 Cephalic eyes are absent in the Scaphopods and in the adult state 

 in Amphineura and the Lamellibranchs (with the exception of the 

 Mytilidae and Avicula). In Molluscs devoid of cephalic eyes, visual 

 organs are often developed on the mantle. Thus among the 

 Amphineura they are found over the whole surface of the mantle 

 in the Chitons ; among the Lamellibranchs on the borders of the 

 mantle only in the Arcidae and in numerous Pectinidae ; finally, one 



