172 



LECTURE XIV. 



however, is the fact ; six well developed ganglions distribute nerves to 

 the muscular fibres of the lobster's tail ; non-ganglionic columns supply 

 the sensitive tail of the hermit-crab. One ganglion, indeed, is present 

 in the Pagurus, but both its situation and office alike militate against 

 the hypothesis of its special subserviency to sensation : it is developed 

 upon the end of the smooth abdominal chords, and seems to have 

 been called into existence solely to regulate the actions of the muscles 

 of the claspers by which the hermit keeps firm hold of the columella 

 of its borrowed dwelling. 



The general progress of the development of the nervous system in 

 the Crustacea has been, as we have seen, attended with increased 

 size, and diminished numbers of its central or ganglionic nerves. 

 The divisions of each pair of ganglia first coalesce by transverse 

 approximation : distinct pairs of ganglia approximate longitudinally, 

 conjoining as usual from behind forwards: confluent groups of ganglia 

 are next found in definite parts of the body, as on the thorax of 

 those species which have special developments and uses for par- 

 ticular legs. In the crab, in which the general form of the body 

 attains its most compact form (^fig' 90.), the ventral nervous trunks 



are concentrated into 

 one large oval gan- 

 glion(^), from which 

 the nerves radiate to 

 all parts of the trunk, 

 the legs, and the 

 short tail. 



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 beau- 

 tiful plates have been 

 ^^"'^'"■- taken. The corre- 



sponding structure of the nervous system is also well displayed by 

 Audouin and Edwards in the Maia. An analogous concentration of the 

 nervous system, but with interesting modifications, has been described 

 by Professor Van der Hoeven*, in the Limulus or King-crab, the 

 most gigantic form of the Entomostracous tribe, and probably the 



Recherches sur I'Anat. des Liiniiles. Fol. ISfiS. 



