18 THE MOLLUSCA 
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 optic nerve. The osphradia 
(Ray Lankester) are situated near the entrance to the pallial cavity 
(Figs. 58 and 89, os; 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 lined 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 supplying 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 ofocrypts, 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 Molluses 
their equilibrium. 
The 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 
