XXX INTRODUCTION. 



two separate nuclei of nervous substance, united by a 

 transverse band, vrith an anterior and posterior nervous 

 filament uniting each to the antecedent and succeeding 

 pairs, the number of ganglia (thirteen) coincides with that 

 of the segments of the body. Proceeding upwards, a 

 condensation, both lateral and longitudinal, of certain of 

 the ganglia is found to be coincident with the concentra- 

 tion of the rings, until in the crab the whole of the abdo- 

 minal and thoracic ganglia become concentrated into one 

 mass, from which the nerves radiate in a most beautiful 

 manner to the parts about the mouth, the limbs, &c. 

 The conclusions to which their elaborate researches have 

 conducted Messrs. Audouin and Milne Edwards are thus 

 given : — 



" Le systeme nerveux des Crustaces se compose tou- 

 jours de noyaux medullaires dont le nombre normal est 

 egal a celui des membres, et toutes les modifications qu'on 

 y rencontre dependent principalement de rapprochemens 

 plus ou moins complets de ces noyaux, agglomeration qui 

 s'opere des cotes vers la ligne mediane en memo temps 

 que dans la direction longitudinale, mais peuvent tenir 

 aussi en partie a un arret de developpement dans un 

 certain nombre de ces noyaux." * 



The organ of hearing is found only in the higher forms 

 of this class. In the larger Decapoda, and particularly 

 in the brachyurous group, it is very easily seen, on re- 

 moving a little crustaceous plate in the basal joint of the 

 second antennae, and thus exposing a small cavity. This 

 operculum is pierced by a small oval opening, covered 

 with a membrane ; and in the Macroura, the whole closure 



* Jlist. Nat. des Crustaces, t. i. p. M7. 



