CRUSTACEA. 



309 



attains most compactness {fig. 131.), the ventral nervous trunks 

 are concentrated into one large oval ganglion (f/), from which the 

 nerves radiate to all parts of 'the trunk, the legs, and the short tail. 



131 



This condition of the nervous system has been described by Cuvier 

 in the common crab, and is illustrated by Mr. Swan's dissections, 

 from which his beautiful plates have been taken.* The correspond- 

 ing structure of the nervous system is also well displayed by Audouin 

 and Edwards in Main. An analogous concentration, but not a 

 homologous one, obtains in Limulus. Here the nervous substance is 

 chiefly massed round the oesophagus, the fore part of the ring 

 expanding into a pair of ganglions from which the nerves are sent off 

 to the small median and large lateral eyes : the nerves to the latter 

 are of great length, Avind round tlie anterior apodemata, and bend 

 back to their termination, breaking up into a fasciculus of minute 

 filaments before penetrating the large compound eye. Two stomato- 

 gastric nerves arise from the upper and fore part of the ring. Vyo\\\ 

 the under surface of the fore part of the ring, a small pair of nerves 

 pass to the first short pair of forcipated jaw-feet : five large nerves 

 proceed from each side of the ring to the five succeeding jaw-feet. 

 A pair of slender nerves pass to the spiny-edged lamelliform appen- 

 dage. The posterior part of the nervous ring is prolonged backwards 

 in the form of a chord, having four ganglionic enlargements on its 

 ventral surface, and terminates opposite the penultimate postabdo- 

 minal plate in a fifth slight ganglionic enlargement which bifurcates : 

 each division sends off a few nerves as it pi'oceeds to the caudal 



* CCXXXIV. Part I. pi. 1. and 2. 



