8 PEOCEEDTNOS OF THE MALACOLOGICAL SOCIETY. 



cilia, which one could lumlly expect to see in material so preserved, 

 the epithelium, instead of consisting of a single row of cells of a 

 slightly larger size than those covering the surrounding parts, is con- 

 siclcrably tliickened at the anterior and posterior margins of the vessel, 

 and in these localities is apparently composed of more than one row 

 of cells, for the nuclei are arranged in two fairly distinct layers — 

 a distal row of regularly disposed nuclei continuous with those of the 

 low epithelium of the gill lamella), and an irregularly scattered . 

 proximal layer (PI. II, Fig. 2, ep.). In the central portion of the 

 blood-vessel the epithelium resumes its low, single-layered condition. 

 The ganglionated nerve-cord lies beneath the anterior tliickening. 



It would be rash to give any deiinite opinion upon the nature of 

 this epithelium, but this much one may say with safety — Its structure 

 is not re])ugnant to the idea that it may possess a sensory function, 

 for the distal row of nuclei may very possibly belong to indilferent 

 supporting cells, and the pro.ximal scattered layer is somewhat 

 suggestive of hair cells packed between the supporting cells ; then, 

 again, the local thickening of the epithelium would be in perfect 

 harmony with such a view. Of course the sensory nature of this 

 epithelium is the vaguest of possibilities, if we consider the epithelium 

 by itself alone ; but when taken in conjunction with the presence of 

 a nerve-cord close beneath it, richly endowed with sensory ganglion 

 cells, the possibility becomes so strong a probability that I think we 

 may with some confidence regard this complex of lofty epithelium and 

 ganglion as a sensory organ, in all probability an osphradium. 



With the hope of grining further knowledge on this point, it 

 seemed desirable to examine the gills of some other species of Chiton 

 which could be obtained in a state more suitable for the microscope ; 

 I therefore procured some specimens of Acanthochites dincrepans 

 (Brown), a fair-sized Chiton occurring among the Channel Islands. 

 In this mollusc the gills are not restricted so entirely to the hinder 

 region as in llanJeija ahyssorvm, but extend backwards from about 

 the middle of the body, giadually increasing in size from the anterior 

 end. "When examined with a pocket lens, there can be seen running 

 down the efferent branchial vessel of each gill a narrow brown line, 

 which recalls to the mind Spengel's brown patch, with this ditterence, 

 however, that whereas his was a difiuse patch, this is a hard narrow 

 line, which is fairly distinct at the base and gradually becomes fainter 

 as it approaches the tip of the gill. 



A transverse section of one of the gills shows that this brown line 

 is due to a narrow ridge of elevated pigmented epithelium surmounting 

 the nerve, and following its course for a considerable distance towards 

 the point of the gill. 



The detailed structure of this epithelium (PL II, Pig. 3, p.ep.), as far 

 as I have been able to make it out, leaves little doubt as to its nature, 

 for it appears to consist of narrow hair cells packed into the inter- 

 spaces between large pigmented supporting cells, a condition highly 

 characteristic of a sensory epithelium. The supporting cells are some- 

 what conical in shape ; the base of the cone is directed towards the 

 e.xterior, and is the portion of the cell richest in pigments. The hair 



