CH. Vl] THE FRESH-WATER MUSSEL 183 



Occasionally, according to Hartog 1 , there is to be found 

 on one or both of the cerebro-visceral connectives a 

 minute ganglion, situated in front of the pericardium at 

 the point where the right and left commissures diverge 

 from one another. This observation I have recently been 

 able to confirm 2 . 



The organs of special sense are few and inconspicuous. 

 The tentacles surrounding the inhalant siphon are un- 

 doubtedly tactile and gustatory in function. They are 

 also keenly sensitive to changes of light and shade, and 

 this in spite of absence of all special optic nerve-endings. 

 Other portions of the body are also sensitive to light, 

 particularly the surface of the foot, as I have proved by 

 repeated experiments in which the siphons were screened 

 from the source of light 3 . Tactile cells, or nerve-endings, 

 whose outer free ends bear fine setae, are abundant on the 

 edges of the mantle, especially near the siphons, and less 

 plentifully upon the labial palps, the inner surface of the 

 mantle and on the foot. An olfactory epithelium, or 

 osphradium, is present covering the ventral surface of the 

 visceral ganglia and extending a short distance along a 

 pair of stout nerves which proceed thence to the gills and 

 contain numerous ganglion cells. The supposed auditory 

 organs, the otocysts, lie a little way behind the pedal 

 ganglia. Each is a small vesicle whose walls are composed 

 of sensory and of ciliated cells ; the cavity of the vesicle is 

 occupied by fluid in which is a single spherical calcareous 



1 Howes, Atlas of Pract. Elem. Zool. 1902. Expl. of Fig. xix. 



PI. XXII. 



2 Latter, Nature, LXVIII. Oct. 1903, p. 623. 



3 Cf. White, Ann. Mag. Nat. Hist. 1869, 



