202 NERVOUS SYSTEM. 



touch each other on the median plain, and the ganglia on the 

 two sides have coalesced to form a single chain along the 

 middle of the abdominal surface of the body. The ganglia 

 are still nearly equidistant, and equally developed along the 

 columns ; but where the minute posterior tapering segments 

 of this animal have advanced and united, their ganglionic 

 matter appears likewise to have been carried forwards to en- 

 large the ninth or terminal ganglion (Fig. 86. B.) The 

 transverse concentration of the columns and ganglia towards 

 the median plain, thus seen in the lowest Crustacea, is suc- 

 ceeded in higher species by a longitudinal movement of the 

 nervous matter directed, as we have seen in inferior classes of 

 articulata chiefly to two points of the body, the head and 

 the thorax, from which the largest and most important ap- 

 pendices of the body, whether for sensation, mastication, or 

 progressive motion are developed. In the long-tailed deca- 

 pods, as the lobster (Fig. 86. C,) and the cray-fish, not only 

 is the sympathetic system of nerves derived from the 

 lateral ganglia of the stomach, greatly developed, as shown 

 nearly twenty years since by Succow, and the ganglia 

 and columns have coalesced and met transversely along the 

 whole body ; but in the region of the thorax, from which 

 the five pairs of large extremities are developed, the ganglia 

 (Fig. 86. C. a. 6,) have both enlarged in size above those of 

 the post-abdomen (C. 6 12,) and considerably approximated 

 to each other in a longitudinal direction. In the higher enh 

 tomoid articulata the segments first coalesce on the anterior 

 and posterior portions of the trunk, and hence the enlarged 

 form presented by their cephalic and caudal ganglia, indepen- 

 dent of the great size often attained by the appendices deve- 

 loped from the terminal parts of the body. The ganglia and 

 columns of the post-abdomen in these macrourous decapods 

 (C. 6. 12,) as shown by Succow, retain much of their primi- 

 tive simple form, like the segments and appendices of that 

 portion of the trunk; but the last ganglion (C. 12,) advanced 

 to the penultimate segment, is here of great size, from the 

 magnitude of the swimming appendices developed from the 

 two caudal segments. The thoracic ganglia and columns are 

 excluded from the general cavity of the trunk, and are en- 

 closed in a distinct canal with solid calcified parietes prolong- 

 ed inwards from the exterior shell. The most concentrated 

 form of the nervous system met with in the Crustacea and its 



