CRUSTACEA. 



339 



tinct. The reader will at once recognise the resemblance be- 

 tween these changes and those already described as taking place 

 during the progress of evolution in the caterpillar: the same 

 great law is, in fact, in operation in both cases, and the same 

 results are obtained from the completion of the process.* 



From a review of the above facts, Milne Edwards and Au- 

 douin arrived at the following conclusions : 1st. That the ner- 

 vous system of Crustacea consists uniformly of medullary nuclei 

 (ganglions), the normal number of which is the same as that of 

 the segments or rings of the body. #. That all the modifications 

 encountered, whether at different periods of the developement or 

 in different species of the series, depend especially on the more or 

 less complete approximation of these nuclei, and to an arrest of 

 developement in some of their number. 3. That approximation 

 takes place from the sides towards the mesian line, as well as in a 

 longitudinal direction. 



Fig. 161. 



(374.) In the Crab the distribution of the nerves is briefly as 

 follows : The encephalic mass, or brain, which still occupies its 



* For a minute account of the arrangement of the nervous system in these animals, 

 the reader is referred to the Cyclop, of Anat. and Phys. art. CRUSTACEA ; by Dr. Milne 

 Edwards. 



z 2 



