$ 272. THE CRUSTACEA. 819 
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 off the two 
esophageal 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 gan- 
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 cesophageal 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 Laemodipoda, 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 Maia squinado. 
U dAudouin and Milne Edwards, Ann. d. 8c. 
Nat. loc. cit. p. 91, Pl. VI., and Milne Edwards, 
Hist. Nat. d. Crust. I. p. 141, Pl. XI. fig. 5, 10 
(Maia squinado and Cancer maenas). 
12 Audouin and Milne Edwards, loc. cit. p. 
79, Pl. IL. fig. 1, and Miine Edwards, loc. cit. p. 
129, PL. XI. fig. 1 (Falitrus). 
13 Straus, Mém. sur les Hiella, in the Mém. du 
Mus. d’Hist. Nat. XVIII. 1829, p. 60, Pl. IV. fig. 
16 (Hyperia). 
14 Treviranus, Verm. Schrift. I. p. 63, Tab. 
- IX. fig. 53 (Porcellio scaber) ; Brandt, Medizin. 
Zool. IL. p. 75, Tab. XV. fig. 28 (Oniscus mura- 
rius), and Rathké, De Bopyro et Nereide, p. 14, 
Tab. III. fig. 4 (Bopyrus squillarum). 
15 Rathké, Danzig. Schrift. loc. cit. p. 127, Tab. 
IV. fig. 2 (Idothea entomon). 
16 Audouin and Milne Edwards, loc. cit. p. 83, 
PI. II. fig. 2, and Mzine Edwards, loc. cit. Pl. XI. 
fig. 2. : 
W Rathké, Nov. Act. Nat. Cur. XX. Pt. I. p. 33, 
Tab. VI. fig. 15. 
18 Lereboullet, Ann.d. Sc. Nat. XX. p. 124, Pl. 
V. fig. 24. 
he Porcellio, Oniscus, Armadillidium, Ido- 
thea. 
2 Treviranus, Vermisch. Schrift. II. p. 8, Taf. 
I. fig. 5, and Roussel de Vauzéeme, Ann. d. Se. 
Nat. I. p. 253, Pl. IX. fig. 19 (Cyamus). 
According to Frey and Leuckart (Beitr. p. 
102), the ventral cord of Caprella somewhat re+ 
sembles that of Cyamus. 
