$ 272. THE CRUSTACEA. 321 
are connected together by double, transverse commissures, which, poste- 
riorly, become single, and, finally, wholly disappear. The longitudinal 
commissures are disposed in a like manner; they are double and wide 
apart in front, but, posteriorly, approximate and are proportionably short- 
ened, until they fuse together, and then entirely disappear, — the cord 
terminating ina simple moniliform band which-ends above the last pair of 
feet. The other abdominal segments which have no feet, receive their 
nerves from two long cords which arise from the twenty-fourth and twenty- 
fifth< abdominal ganglia and accompany the intestinal canal to the last. 
segment of the tail, where they end in a ganglionic enlargement from 
which are given off several short filaments, beside a long nerve to the two. 
caudal bristles. In the other Phyllopoda, the nervous system is observed 
with difficulty, probably from its tenuity; and, as yet, only a single flat- 
tened cephalic ganglion has been found.“ With the very small Lophyro- 
poda, these difficulties are even greater, for here there has been observed a 
multi-constricted, nervous mass, situated in front of the cesophagus, which 
may be regarded as a cerebral ganglion, since it sends off, in front, several 
filaments to the tactile and ocular organs; and behind, two cords which 
surround the cesophagus, and join, perhaps, in an abdominal ganglion, 
Among the Siphonostoma, with Argulus, as with the Lophyropoda, 
the nervous ‘centre is reduced to a cerebral mass situated above the pro- 
boscis, — and composed of three ganglia arranged triangularly. With 
the other parasitic Crustacea, of which the head and organs of sense have 
gradually disappeared, the cerebral ganglion always becomes correspond- 
ingly less apparent, while the abdominal cord is the more distinct. This 
is so with the genus Chondracanthus, which has a cerebral ganglion, and in 
the few segments of the body, several widely separated (laterally) ganglia 
connected together by longitudinal, double commissures. With Diche- 
27 Brongniart, loc. cit. p. 87, Pl. XIII. fig. 2, 
8, a. (Limnadia), and Joly, loc. cit. p. 310, Pl. V. 
fig. 5, k. and Pl. VIII. fig. 21, a. isanra). This 
last naturalist has been unable to find a cerebral 
ganglion with Aréemia (loc. cit. p. 242).* 
28 An analogous brain, divided by constrictions 
into three ganglia placed in a row, has been fig- 
ured by Schaffer (Die zackigen Wasserflihe, loc. 
* [§ 272, note 27.] The investigations of Ley- 
dig (loc. cit. Stebold and Kelliker’s Zeitsch. III. 
p. 290) have shown that, with at least Artemia and 
Branchipus of the Phyllopoda, the nervous system 
is well developed. This system seems, for the most 
part, to have escaped the observation or former in- 
vestigators from want of manipulation; Leydig 
has described it with detail, and divides it, as 
usual, into a central and a peripheric portion. The 
first consists of the brain which sends off nerves to 
the organs of sense (eyes, antennae, &c.) and con- 
nects, by two commissural cords which embrace 
the cesophagus, with the ventral cord. This cord 
is composed of eleven (Branchipus), or twelve 
(4rtemia) ganglia, which are connected, succes- 
sively, by two longitudinal commissures, and, lat- 
erally, each, by a double, transverse commissure. 
Each of these ganglia sends off, from its outer 
border, three nerves which are distributed to the 
abdominal] organs and appendages, and to the skin. 
— Ep. 
cit. p. 39, Tab. IL. fig. II. 1, 2, 3), by Straus (loc. 
cit. p. 396, Pl. XXIX. fig. 6, b. d. e. (Daphnia)),, 
and by Lovén (loc. cit. p. 151, Taf. V. fig. 5, d. 
(Evadne)). 
2 Jurine, Ann. du Mus. VII. p. 447, Pl. 
XXVLI. fig. 11, and Vogt, loc. cit. p. 14, fig. 1, L., 
11.t 
80 Rathké, Nov. Act. Nat. Cur. XX. p. 125. 
+ [| § 272, note 29.] The recent researches of 
Leydig (loc. cit. Siebold and Kélliker’s Zeitsch. 
II. p. 328) have extended our knowledge of the 
nervous system with these lower Crustacea. In 
Arguluys, this observer found the central nerv- 
ous system to consist of a cerebral portion and a 
ventral cord. The first, or brain, is composed of 
two parts—one anterior and club-shaped, the 
other, beneath the first, pyriform and much the 
larger. This portion connects, by two commis 
sures which embrace the cesophagus, with the 
ventral cord. This cord is composed of six gan- 
glia. He observed the following distribution of the 
peripheric portion of the nervous system. From 
the brain arise the optic nerves, and behind these, 
two pairs of nerves for the antennae ; of the ven- 
tral ganglia, the first, third and sixth give off 
nerves to the appendages of the body and its 
internal organs. Leydig found no trace of a 
splanchnic system with these animals. — Ep. 
