VII MOLLUSGA—THE SENSORY ORGANS 165 



Siphoniata ; these lie on the retractor muscles of the siphons and at the base of the 

 branchial siphon. These pallial sensory organs also are innervated by the posterior 

 pallial nerves, and may correspond with the anal sensory organs of the Asiphonia/a. 

 Their function is unknown, but is supposed to be analogous to that of tlio 

 osphradia. 



(e) Olfactory Organs of the Chitonidse. 



In the mantle furrow of the Chitonidce there are epithelial sensory 

 organs which are considered to be olfactory. These are ridges and 

 prominences with extraordinarily high epithelium, consisting of 

 glandular cells and thread-like sensory cells. In Chiton Icevis and 

 C. cajetanus there are, on each side of the mantle furrow, two sensory 

 ridges extending along the whole length of the row of gills ; one of 

 these, the parietal ridge, belongs to the outer wall of the furrow, 

 while the paraneural ridge runs along the base of the furrow, above 

 the bases of the gills and under the pleuro-visceral cord. The para- 

 neural ridge is continued a short distance along the inner surface of 

 each gill, so that each gill has an epibranchial sensory prominence. In 

 front of the first pair of gills and near the last the sensory cells in the 

 paraneural ridge become far more numerous in comparison with the 

 glandular cells. Chiton siculus, C. Polii, and Acanthochiton (in which 

 the numerous gills reach far forward) have no parietal and paraneural 

 ridges. The sensory epithelium in these animals is confined to two 

 prominences, paraneural in position, behind the last pair of gills, 

 and connected with a high epithelium covering the pallial wall of 

 the most posterior part of the furrow. 



All these sensory epithelia seem to be innervated from the 

 pleuro-visceral cords. 



The question as to the relation of tliese sensory epithelia in the Ohitonidce to the 

 osphradia of other Molluscs, which here presents itself, is difficult to answer. In 

 position the osphradia best con'espond with the epibranchial prolongations of the 

 paraneural ridges in Chiton Icevis and C. cajetanus. 



3. The " Lateral Organs " of the Diotoeardia. 



At the bases of the epipodial tentacles of Fissurella and the 

 Trochidce, and at the base of the lower tentacles of the epipodial ruff 

 of Haliotis, and also in other parts near the ruff, sensory organs are 

 found which have been compared with the lateral organs of Annelids. 

 They consist of patches of sensory epithelium, which may form 

 either spherical projections or pit-like depressions. The epithelium 

 of these sensory organs which lie at the lower side of the bases of the 

 epipodial tentacles, consists of sensory cells, each of which is provided 

 with a sensory seta, and pigmented supporting cells. Each of these 

 sensory organs is innervated by the nerve of the tentacle near it, 

 which nerve originates in the pedal cord and forms a ganglion in the 

 base of each epipodial tentacle. 



