On the Nervous System of' the Crustacea. 333 



the different periods of its life. The corollaries which are de- 

 duced from the facts elicited by each of these methods of inves- 

 tigation always facilitate the study of organization ; but this ad- 

 vantage is not the only one that may be derived from researches 

 directed toward this object : they sometimes lead us to general 

 principles Avhich seem to be so many laws that regulate the or- 

 ganic formations. It even happens, when these principles are 

 the just expression of truth, that they cause us to anticipate the 

 existence of facts still unknown, and that opinions drawn from 

 deductions of this kind afterwards receive an entire confirmation 

 from direct observation. What we are about to say of the 

 nervous system of the Crustacea affords a striking example of 

 this. 



In a memoir which we presented to the Academic des Sci- 

 ences in 1827 (Annates des Sciences Naturelles, t. xiv. p. 77)^ 

 we endeavoured to make known the different modifications of 

 the nervous system of the Crustacea, and we tried to apprehend 

 the relations existing between the different forms under which 

 it presents itself in that class of animals. We shewed that 

 sometimes there exist two ganglionary chains distinct from each 

 other, and similar to themselves in the whole length of the body ; 

 that, at other times there is but a single chain whose structure 

 is equally uniform ; that, in certain species, a cephalic ganglion 

 and a medullary ring contained within the thorax are alone met 

 with ; and, lastly, that frequently this latter portion of the ner- 

 vous system is replaced by a solid node. At first consideration, 

 one would be led to suppose that the nervous system of each of 

 these animals, having an aspect so different, is formed of ele- 

 ments, which cannot be strictly compared with each other ; but, 

 in pursuing the examination of these parts in a great number of 

 Crustacea, we have met with intermediate states which have 

 enabled us to understand that these dissimilarities depend only 

 upon a series of modifications, consisting in the various degrees 

 of approximations and of centralization, of certain similar parts, 

 or in the want of development of some others. 



This result is in perfect accordance with the principles which 

 M. Serres deduced from his inquiries into the nervous system 

 of other animals, and into cmbryogeny in general. He was led 

 to conclude that this tendency to centralization was one of the 



