372 SEA-SIDE STUDIES. 



when tlie cuticle is not pierced. The needle presses 

 on the cuticle, and the pressure is communicated from 

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

 pressure may be lateral as well as perpendicular, tf 

 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 inquu-e whether they can hear. As 

 in the former case, the answer must depend on what 

 is meant by " hearing.'' If every 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 Pleuro- 

 hranclius : instead of the complex structure found in 

 higher animals, we find a microscopic vesicle contain- 

 ing pebbles suspended in liquid. In the Doris this 

 vesicle has no nerve, but lies upon the cerebroid gan- 

 glion, immediately behind the optic ganglion. Nor 

 have I, in a dozen dissections, been able to detect a 

 nerve in the Pleurohranchus, although Krohn describes 

 one in the sub-genus Pleurohranchcea. At any rate, 

 embryology proves the nerve to be a subsequent addi- 

 tion, since in the embryos of all the Nudibranchs the 

 ear is a simple vesicle containing a single otolithe, with 



