§212. 



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 off, 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 pai-asitic 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 Choiidr acanthus, 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 Brongninrt, loc. cit. p. 87, PI. XIII. fig. 2, 

 3, a. (Limnadia), and Joly, loc. cit. p. 310, PI. V. 

 fig. 5, k. and PI. VIII. fig. 21, a. (haura). This 

 last naturalist has been unable to find a cerebral 

 ganglion with Artemia (loc. cit. p. 242).* 



28 An analogous brain, divided by constrictions 

 into three ganglia placed In a row, has been fig- 

 ured by Sch offer (Die zackigen Wasserflohe, loc. 



* [§ 272, note 27.] The investigations of Ley- 

 dig (loc. cit. Siebold and KolUker^s Zeitsch. III. 

 p. 290) have shown that, with at least Artemia and 

 liranckipus of the Phyllopoda, the nervous system 

 is well developed. This system seems, for the most 

 part, to have escaped the observation of 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 oesophagus, with the ventral cord. This cord 

 is composed of eleven (Branckipun), or twelve 

 (Artemia) 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. 

 — Ed. 



cit. p. 39, Tab. II. fig. II. 1, 2, 3), by Straus (loc. 

 cit. p. 396, PI. XXIX. fig. 6, b. d. e. {Daphnia% 

 and by Lovin (loc. cit. p. 151, Taf. V. fig. 5, d. 

 (fivadne)). 



2fl Jitrine, Ann. du Mus. Vn. p. 447, PI. 

 XXVI. fig. 11, and Fogt, loc. cit. p. 14, fig. 1, L., 

 ll.t 



30 Rathki, Nov. Act. Nat. Cur. XX. p. 125. 



t I § 272, note 29.] The recent researches of 

 Leydig (loc. cit. Siebold and Kblliker''s Zeitsch. 

 II. p. 328) have extended our knowledge of the 

 nervous system witli these lower Crustacea. In 

 Argulus, 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 oesophagus, 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 at 

 splanchnic system with these animals. — Ed. 



