306 LECTURE XIV. 



The neurology of the Crustacea has been most completely illus- 

 trated 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 Astacus fiuviatilis {Jig. 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 oi'gans 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 

 are developed on the abdominal chords, which are continued along 

 the muscular tail : the last, which is above the anus, is the largest, 

 and radiates the nerves to the terminal swimming plates of the tail. 

 This is probably a coalescence of the originally distinct ganglions of 

 the sixth and seventh segments of the abdomen or tail. 



The nervous system of the lobster is well displayed in dissections 

 by John Hunter*, and has been beautifully illustrated by INIr. Swan.f 

 The nei'ves of the lobster closely correspond with those of the fresh- 

 water species, or craw-fish. The non-ganglionic tracts are shown in 

 a dissection of the lobster by Mr. Newport | ; but the distinctions of 

 the origins of the nerves from the dorsal and the ventral tracts are, 

 as Mr. Swan remarks, by no means clear. The oesophageal columns 

 are united in both species of Astacus by a transverse commissural 

 chord. 



In the prawn {Palemoii) and rock-lobster {Palinurus), the thoracic 

 ganglia coalesce to form a long, elliptical, perforated nervous mass. 

 In the hermit-crab {Pagurtis)^, the cephalic ganglion presents 

 a transversely quadrate form, and sends off the usual nerves to the 

 eyes, the ears, and the antennae. The lateral oesophageal chords, 



* Preps. Nos. 1301, 1302, 1303, described in X. vol. iii. p. 15. 

 t CCXXXIV. Part I. pi. 3. and 4. J Prep. No. 1302, A. 



§ Prep. No. 1303. B. 



