§ 272. THE CRUSTACEA. 319 



Decapoda, to the organs of sense; the second is large, round or oval, and 

 situated in the centre of the thorax, — it is sometimes perforated cen- 

 trally,""* and supplies all the nerves of the trunk, beside sending oif the two 

 oesophageal cords. These cords are connected by the transverse commissure 

 already mentioned, and give off filaments to the organs of mastication. 

 From the posterior extremity of the cord, there arises a simple nervous 

 trunk, free from ganglia, and extending along the median line to the very 

 extremity of the tail.'"' 



Among the Amphipoda, the Gammarina have a brain scarcely larger 

 than the first of the abdominal ganglia ; these last, twelve in number, 

 are connected by double commissures, and the posterior ones belonging to 

 the segments of the body which have false feet, are always smaller than 

 the others.''-' With the large-headed Hyperina, the two cerebral ganglia 

 are considerably larger than the abdominal ones, which are ten in number, 

 and of unequal size. Their commissures are contiguous, and the first g-an- 

 glion, which is the largest, is probably the result of the fusion of two 



pairs 



(!•■!) 



With the Isopoda, the abdominal cord, which is connected with the cere- 

 bral ganglia by two short oesophageal cords, is composed of seven pairs of 

 ganglia, situated in the thoracic and anterior abdominal segments, and con- 

 nected together, successively, by double commissures. In some genera, the 

 posterior ganglia send off radiating nerves to the partially abortive and 

 partially fused terminal segments.'"' In others, these seven pairs are 

 succeeded by five or six pairs of others, smaller, and which, with Idothea, 

 are connected together by double commissures ;'''*' but with the genera 

 Cymothoa,^^^'" Aega^^'^ and Lygidium,^^'*^ are contiguous. With many Iso- 

 poda, the peripheric nerves are given off, not only from the ganglia, but 

 also from their longitudinal commissures, and the posterior ones are distrib- 

 uted to the dorsal region of the animal. ''"' 



With the Laemodijjoda, the abdominal cord is composed of eight pairs 

 of ganglia, of which the first two are situated in the cephalic segment, one 

 behind the other, and send off nerves to the organs of mastication, and to 

 the first pair of feet, — thus corresponding to the result of the fusion of the 

 first two thoracic segments with the head. The other pairs are connected 

 by very distinct double commissures, which, between the last two pairs, are 

 quite short, thus bringing the last three pairs almost together in the third 

 terminal segment of the body.'-"' 



With the Myriapoda the ganglia of the abdominal cord are very numer- 

 ous, and nearly all of the same size. The brain consists, usually, of a 



10 Mail nqninndo. 16 Audouin and Milne Edwards, loc. cit. p. 83, 



11 Audouin and Milne Edwards, Ann. d. Sc. PI. II. fig. 2, and Milne Edwards, luc. cit. PI. XI. 

 Nat. loc. cit. p. 91, PI. VI., and Milne. Edwards, fig. 2. 



Hist. Nat. d. Crust. I. p. 141, PI. .XI. fig. 5, 10 17 /J«/H<?, Nov. Act. Nat. Cur. XX. Pt. I. p. 33, 



{Maia squinado and Cancer maenas). Tab. \l. fig. 15. 



i'^ Audouin and Milne Edwardi, \oc. cit. p. 18 />e;e6ou//e<, Ann. d. Sc. Nat. XX. p. 124, PI. 



79, PI. II. fig. 1, and Milne Edwards, loc. cit. p. V. fig. 24. 



129, PI. XI. fig. 1 (ra/j7ri«). li» Porcellio, Oniscus, Armadillidium, Ido- 



13 Straus, Mem. sur les lliella, in the Mem. du ttiea. 



Mus. d'llist. Nat. XVIII. 1829, p. 60, PI. IV. fig. Li» Treviranus, Vermisch. Schrift. II. p. 8, Taf. 



16 (Hi/peria). I. fig. 5, and Roussel de Vauzime, Ann. d. So. 



H Treviranus, Verm. Schrift. I. p. 63, Tab. Nat. I. p. 253, PI. IX. fig. 19 (Cyamus). 



IX. tg. 5'i (Porcellio scaber) ; Brandt, Medizin. According to Frey and Leuckart (Beitr. p. 



Zool. II. 1). 75, Tab. XV. fig. 2S (Oni.<icus inura- 102), the ventral cord of CaprelLa somewhat ro- 



rins), and Rathki, l)e Bopyro et Nereide, p. 14, sembles that of Cyamus. 

 Tab. III. fig. 4 {Bopyrus squillarum). 



15 Rathki, Danzig. Schrift. loc. cit. p. 127, Tab. 

 IV. fig. 2 {Idothea entomon). 



