318 THE CRUSTACEA. § 272. 



together laterally, on the median line. Of these, the first six pairs, belong, 

 ing to the thoracic and to the anterior abdominal segments, are the larger, 

 and send off nerves principally to the foot-jaws, to the prehensile, and to the 

 ambulatory organs. The two longitudinal commissures between the anterior 

 abdominal ganglia, are separate; but those between the posterior ganglia are, 

 on the contrary, blended into a single cord. In some species, these com- 

 missures are wholly wanting between the anterior ganglia. ^'^^ The brain 

 consists of a single transverse ganglion ; from its front and sides pass off 

 several nerves for the antennae, the olfactory organs, the eyes, and the 

 auditory organs ; while, from behind, it sends off the two long cords which 

 surround the oesophagus. These last give branches on their course to the 

 organs of mastication, and interanastomose behind the oesophagus, just 

 betbre reaching the first thoracic ganglion, by a transverse filament. '^> 



With the Stoniapoda, the nervous system is composed of a cei-ebral gan- 

 glion, and of about ten abdominal ganglia ; of these, with the Squillina, the 

 last six belong to the tail, while the remaining four, anterior, send nerves to 

 the thorax, and to the first three abdominal segments. The size of the first, 

 which sends nerves to the prehensile feet, is due to its being composed 

 of several ganglia fused together. ^^^ With the Mysina, the five or six 

 largest ganglia belong to the thorax and to the anterior part of the abdo- 

 men, and are connected together by short, double commissures.*"' 



The genus Phyllosoma has two extraordinarily long and very small 

 oesophageal cords extending from the brain to the abdominal cord. The 

 thoracic portion of this last is composed of three pairs of gangli^i, blended 

 almost into a single mass ; these are succeeded by six pairs of large ab- 

 dominal ganglia, arranged in two longitudinal rows, and interconnected by 

 six very short, transverse filaments. In the short tail, there are, more- 

 over, six pairs of ganglia, smaller and laterally contiguous, but connected 

 successively by very small longitudinal filaments.'^' 



With the Anomoura, which resemble the Brachyura in the abortion of 

 the post-abdomen, the structure of the nervous system confirms this affin- 

 ity. With Pagurus, the anterior portion of the abdominal cord consists 

 only of three ganglia, which send nerves to the foot-jaws, to the cheliform, 

 and to the partially abortive ambulatory feet. But the posterior part of 

 this cord consists of two cords which arise from the third abdominal gan- 

 glion, and unite, just in front of the arms, in the fourth and last gan- 

 glion.*''' With the genus Homola, the five pairs of the anterior abdomi- 

 nal ganglia are fused into a single mass which is perforated through its 

 centre. From the posterior border of this mass a simple nervous cord 

 as rudiment of the posterior part of the ventral cord, passes oft' to the wholly 

 abortive post-abdomen.*^' 



With the Brachyura, the nervous system has only two central masses, 

 one cerebral, the other abdominal. The first sends nerves, as in the other 



3 Palinurux, and Palaemon (Andouin and 5 Cuvier, Lecons &c. III. 1845, p. 330, and 



Milne Eiiv^rd^, loc. cit.). lu these two Crusta- Delle Chiaje, Desoriz. &c. Tav. LXXXVI. fig. 



cea, there is only a small fissure in the centre of 5. 



the principal ganglionic mass, after the fusion of G Frey, De Mysidis flexuosae anat. p. 9. 



the ganglia. 7 Audouin and Milne Edwards, Ann. d. Sc. 



i This transverse filament is absent neither with Nat. loc. cit. p. 81, PI. III. 



Palaemon, Palimtrus, nor with Homarus, and >i Ciivier, Liogjns, &c., loc. cii. p. 329; and 



Astacus. It was overlooked by Sackow in the Owen, Lect. on Comp. Anat. p. 170. 



crawfish, although distinctly seen by Brandt ; see 9 Milne Edwards, Hist. Nat. d. Crust. PI. XI. 



his Mediziii. Zool. loc. cit., and his Bemerkungen fig. 9. 

 liber die Mundmagen-nervcu, loc. cit. Tab. I. fig. 

 1, 2 K, or Ann. d.'^Sc. Nat. V. 1836, PI. IV. 



