CRUSTACEA. 169 



verse, and connects the two ganglions of each pair with each other, 

 and with the transverse nerves. The Talitrus presents ten pairs of 

 nearly equidistant sub-abdominal ganglions, the two first and the two 

 last being most approximated. In the Cymotlioa {fig- 89.), a species 

 in which the tapering terminal segments of the body have begun to 

 be concentrated by longitudinal approximation, the corresponding 

 nervous ganglions at the posterior part of the abominal 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 con- 

 fluence. The ganglions of each lateral abdominal chord have 

 now more completely coalesced by transverse approximation. In 

 the Squilla mantis, the supra-cesophageal ganglion or brain sends 

 off five nerves on each side, those to the long antennae being recurrent 

 in their course. Stomatogastric nerves arise from the oesophageal 

 chords, which unite below into a long sub-oesophageal ganglion, 

 apparently formed by a confluence 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 mus- 

 cular tail. 



The neurology of the Crustacea has been most completely illustrated 

 in those species which are covered by a dense insensible crust. 

 Succow, in 1818, and Brandt, in 1833, published excellent descrip- 

 tions of the nervous system of the Astacus fluviatilis {fig. 3. p. 13.). 

 The cephalic ganglion sends branches to the eyes, to the large and 

 small antennae, to the antennal sheaths, and to the organs of hearing. 

 A nearly straight chord is continued from this ganglion, on each side 

 of the oesophagus, to the first of the sub-abdominal series. An 

 azygous nerve arises from the middle of the posterior surface of the 

 cephalic ganglion, and passes backwards to the stomach, where it 

 communicates with two nerves given off one from each of the oeso- 

 phageal chords to supply the stomach. The sub-oesophageal ganglion 

 distributes nerves to the masticatory organs and to the pharynx, just 

 as the medulla oblongata sends off the fifth pair and glosso-pharyn- 

 geal in the vertebrate animals. The second to the sixth thoracic 

 ganglia inclusive, supply the feet and gills with nervous influence ; 

 the generative organs receive long filaments from the fourth, the 

 fifth, and the sixth thoracic ganglions. The ventral or sternal artery, 

 descending from the heart, passes between the nervous columns at 

 the interspace between the fourth and fifth ganglions. Six ganglions 



