§ 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 in a 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 oesophagus, which 

 may be regarded as a cerebral ganglion, since it sends oft", in front, several 

 filaments to the tactile and ocular organs ; and behind, two cords which 

 surround the oesophagus, 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 Ckondracanthus, 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 Dicke- 



27 Brongniart, loc. c!t. p. 87, PI. XIII. flg. 2, cit. p. 39, Tab. 11. fig. II. 1, 2, 3), by Straus (loc. 



3, a. (Limnadia), and Joli/, loc. cit. p. 310, PI. V. cit. p. 396, PI. XXIX. fig. 6, b. d. e. (Daphnia)), 



fig. 5, k. and PI. VIII. fig. 21, a. (Isaura). This and by Lovdn (loc. cit. p. 151, Taf. V. fig. 5, d. 



last naturalist has been unable to find a cerebral {Evadne)). 



ganglion with jJr^emja (loc. cit. p. 242).* '-^9 Jurine, Ann. du Mus. VII. p. 447, PI. 



2S An analogous brain, divided by constrictions XXVI. fig. 11, and f^offt, loc. cit. p. 14, fig. 1, Ii,, 



into three ganglia placed in a row, has been fig- 11. t 



m-ed by Schaff'er (Die zackigen Wassertiöhe, loc. 30 Raihk^, Nov. Act. Nat. Cur. XX. p. 125. 



* [ § 272, note 27.] The investigations of Leij- t I § 272, note 29.] The recent researches of 



dig (loc. cit. Siebotd and Kolliker's Zeitsch. III. Leydig (loc. cit. Siebold and Kölliker\^ Zeitsch. 



p. 290) have shown that, with at least Artemia and II. p. 328) have extended our knowledge of the 



Branchipus of the Phyllopoda, the nervous system nervous system with these lower Crustacea. la 



is well developed. This system seems, for the most Argulus, this observer found the central nerv- 



part, to have escaped the observation of former in- ous system to consist of a cerebral portion and a 



vestigators from want of manipulation ; Leydig ventral cord. The first, or brain, is composed of 



has described it with detail, and divides it, as two parts — one anterior and club-shaped, the 



usual, into a central and a peripheric portion. The other, beneath the first, pyriform and much th& 



first consists of the brain which sends off nerves to larger. This portion connects, by two commis 



the organs of sense (eyes, antennae, &c.) and con- sures which embrace the oesophagus, with the 



nects, by two commissural cords which embrace ventral cord. This cord is composed of six gan- 



the oesophagus, with the ventral cord. This cord glia. He observed the following distribution of the- 



is composed of eleven (Branchipus), or twelve peripheric portion of the nervous system. From 



(Artemia) ganglia, which are connected, succes- the brain arise the optic nerves, and behind these,, 



sively, by two longitudinal commissures, and, lat- two pairs of nerves for the antennae ; of the ven- 



erally, each, by a double, transverse commissure, tral ganglia, the first, third and sixth give o£f 



Each of these ganglia sends oflf, from its outer nerves to the appendages of the body and its 



border, three nerves which are distributed to the internal organs. Leydig found no trace of a 



abdominal organs and appendages, and to the skin, splanchnic system with these animals. — Ed. 

 — Ed. 



