'338 



CRUSTACEA. 



masses each, the first and last pairs are united into single ganglia. 

 "As we rise to higher forms the coalescence still proceeds : all the 

 pairs of ganglia soon become united in a transverse direction, and 

 gradually the whole chain becomes shorter by the confusion of 

 several pairs into larger and more powerful masses. 



In the Crab, which, from its terrestrial habits, holds a position 

 among the Crustacea equivalent to that which Spiders occupy 

 among other Articulata, this centralization is carried to the utmost 

 extent ; and all the abdominal and thoracic ganglia become agglo- 

 merated into one great centre, from which nerves radiate to the 

 parts of the mouth and instruments of locomotion (Jig. 160). 



(373.) But this change pig. 160. 



in the condition of 

 the nervous system 

 is not only observ- 

 able as we proceed 

 from species to spe- 

 cies, as they rise 

 higher in the scale 

 of developement ; si- 

 milar phenomena are 

 met with in watch- 

 ing the progress of 

 any individual be- 

 longing to the more 

 perfect families, as it advances from the embryo to its ma- 

 ture condition. Thus in the Cray-fish (Astacus fluviatilis), 

 Rathke* observed, that, when first perceptible, the nervous system 

 consisted of eleven pairs of ganglia, perfectly distinct from each 

 other, and situated on each side of the mesial line of the body. 

 The six first pairs then unite transversely, so as to form as many 

 single masses, from which the nerves of the mandibles and foot- 

 jaws emanate ; while the five posterior, from which the nerves of 

 the ambulatory extremities are given off, remain separate. Such is 

 the state at birth, or on leaving the egg ; but further changes occur 

 before the Cray-fish arrives at maturity. The four anterior gan- 

 glia, which supply nerves to the mandibles and foot-jaws, are, by 

 degrees, all consolidated into one mass, and the fifth and sixth 

 likewise coalesce, while the other pairs continue permanently dis- 



* Untersuchungen liber die Bildung des Flusskrebses in the Annales des Sciences 

 Nat. torn. xx. 



