CROSTACEA. 305 



non-ganglionic nervous columns of the Articulata. The same 

 hypothesis will be more severely tried by the comparative anatomy 

 of the nervous system in the higher species of the Crustacea, on 

 account of the varying conditions as to sensation and motion which 

 different parts of their more diversified forms of body present. 



We find in the lowest Isopoda, as the woodlouse (^Oniscus) and 

 the sandhopper {Talitrus), that the supposed sensitive organs, the 

 ganglions of the abdominal chords, are more developed than in the 

 highest Anellides : they are likewise more distinctly bilobed ; each 

 lateral chord presenting its own ganglionic enlargements, which are 

 in juxtaposition, but not confluent, so that there is a distinct pair of 

 ganglia for each segment. If these ganglions be microscopically 

 examined, they present, as their essential structure, large round or 

 pyriform cells with a large nucleolated nucleus ; and three orders of 

 nervous filaments may be perceived in them. The first is longi- 

 tudinal, and extends over the dorsal aspect of the ganglia; the 

 second is also longitudinal, and originates from, or terminates in, 

 the ganglia ; the third is transverse : these fibres pass round the 

 ganglionic cells, and emerge laterally : they, in Talitnts, connect the 

 two ganglions of each pair with each other, and in all Crustacea 

 form the transverse or peripheral nerves. Talitrus presents ten 

 pairs of nearly equidistant sub-abdominal ganglions, the two first 

 and the two last being most approximated. In Cymothoa {Jig. 129.), 

 a species in which the tapering terminal segments of the body have 

 begun to be concentrated by longitudinal approximation, the corre- 

 sponding nervous ganglions at the posterior part of the abdominal 

 chord present a corresponding change, advancing forwards like 

 the caudal part of the spinal column in the metamorphosed larva of 

 the frog. 



In the higher Crustacea, with a thorax covered by the cephalic 

 shield, and supporting disproportionately large and prehensile anterior 

 extremities, the thoracic ganglia exhibit proportionate increase of size, 

 with a tendency to unite, or with actual confluence. The gan- 

 glions of each lateral abdominal chord have now more completely 

 coalesced by transverse approximation. In Squilla mantis, the 

 super-oesophageal ganglion or brain sends off five nerves on each side, 

 those to the long antennae being recurrent in their course. Stomato- 

 gastric nerves arise from the oesophageal chords, which unite below 

 into a long sub-oesophageal ganglion, apparently formed by a con- 

 fluence of three originally distinct pairs. This is succeeded by three 

 other ganglions in the thorax, supplying three pairs of thoracic legs ; 

 and there are six ganglions in the muscular tail. The oesophageal 

 chords are unusually long and slender in the transparent Phyllosoma. 



X 



