NERVOUS SYSTEM. 203 



highest condition presented by this articulated division of the 

 animal kingdom is that found in the short and broad trunks 

 of the brachyourous decapods (Fig. 86. D,) where all the 

 symmetrical ganglia of the columns are generally col- 

 lected into two masses, the one in the head, and the 

 other in the centre of the cephalo-thorax, and where the mo- 

 tor and sensitive columns are almost confined to a nervous 

 band around the wide oesophagus. The anterior of these, or 

 the supra-cesophageal ganglion (86. D. 1,) is comparatively 

 small in the brachyourous decapods, from the smalmess of 

 the cephalic appendices, which it supplies with nerves. 

 The infra-cesophageal nervous mass (86. D. 2,) is of great 

 size, consisting of the whole chain of ganglia, which was 

 originally extended along the body behind the oesophagus, 

 and is favourably situated between the haunches of the legs, 

 under a strong internal osseous arch, in the centre of the 

 trunk. It sends out numerous branches to the surrounding 

 viscera, and to the five pairs of legs which radiate from 

 around that point, and the columns are prolonged backwards 

 ramifying along the short slender post-abdomen, as a simple 

 nervous chord. There are many intermediate conditions of 

 these nervous columns and ganglia between those of the asta- 

 cus (Fig. 86. C,) and of the maia (Fig. 86. D,) some of 

 the macrourous decapods having the thoracic ganglia much 

 more approximated than in the former, and many of the bra- 

 chyourous species having them less concentrated into a mass 

 than in the latter, and similar links are observed to connect 

 together the typical forms of this system in the other orders 

 of Crustacea, and throughout all the articulated classes. Thus 

 the most elevated form of the nervous axis met with in 

 this division of the animal kingdom begins its development 

 with two simple abdominal filaments, like the lowest hel- 

 minthoid form of entozoa, and by a gradual process of con- 

 centration proceeding transversely and longitudinally from 

 the peripheral to the central parts, it arrives at the cyclo-gan- 

 gliated character of the molluscous classes, with its great 

 symmetrical ganglia confined to the resophageal ring. 



