354 SEA-SIDE STUDIES, 



from the cuticle to tlie nerve ; and it is evident that this 

 pressure may be lateral as well as perpendicular. If a nerve 

 be within the range of this lateral pressure, it will be affected ; 

 and although those parts which are liberally supplied with 

 nerves are necessarily more sensitive than others, because 

 more filaments come within the range of lateral pressure, yet 

 no part of the skin is insensible, because no part is without 

 the range of a nerve. 



Having ascertained that our Molluscs cannot see, we have 

 now to inquire whether they can hear. As in the former 

 case, the answer must depend on what is meant by " hearing." 

 If eveiy sensation of light and darkness is to be called sight, 

 and every sensation of sound is to be called hearing, our 

 friends certainly both see and hear — as blind men see, and 

 deaf men hear. Let us examine the organ in a Doris or 

 Pleurohranchus : instead of the complex structure found in 

 higher animals, we find a microscopic vesicle containing 

 pebbles suspended in liquid. In the Doris this vesicle has 

 no nerve, but lies upon the cerebroid gangUon, immediately 

 behind the optic ganglion. Nor have I, in a dozen dissec- 

 tions, been able to detect a nerve in the Pleurohranchus, 

 although Krohn describes one in the sub-genus Pleuro- 

 hranchcea. At any rate, embryology proves the nerve to be 

 a subsequent addition, since in the embryos of all the Nudi- 

 branchs the ear is a simple vesicle containing a single otolithe, 

 with neither nerve nor ganglionic attachment. The mention 

 of embryological indications reminds me that Von Siebold 

 has shown the close analogy which exists between the 

 permanent organ of hearing in the gasteropod j\[olluscs, 



