S20 THE CRUSTACEA. >§. 272. 



distinct right and left half, upon each of which is a kind of Gaagliort 

 opticum, of a size proportionate to that of the development of the eyes. 

 With the Chilopoda, the abdominal ganglia are widely separated from 

 each other, but connected by double commissures which are closely ap- 

 proximated, and in some cases, fused together as a ventral cord.'-'* With 

 Lithobius, and Scittigera, there are sixteen pairs of these ganglia ; with 

 Scolopendra, twenty-two, and with Genphilus, fifty to one hundred and 

 forty. Of these ganglia, the first pair, belonging to the two anterior feet, 

 which are changed into prehensile or tactile organs, are umch the largest. 

 The size of the others corresponds, for the most part, to the development 

 of the feet. <"> 



Of the Chilognatha, the genus Polydesmus, the long feet of which are 

 widely separated, is allied to the preceding section of Myriapoda. Above 

 each two pairs of feet, the abdominal cord is enlarged into two successive 

 ganglia, and the medullary mass between them corresponds to a simple 

 longitudinal commissure.'^' With the other Chilognatha, of which the 

 pairs of feet are close together, the longitudinal commissures are wholly 

 "wanting, so that the ganglia, of a number corresponding to that of the 

 pairs of feet, form a moniliforra cord ; and in some Julidae, the constric- 

 tions of this last are entirely effaced. '-''' 



The disposition of the nervous system of Limulus is remarkable. Its 

 principal mass surrounds the mouth like a ring. From the anterior por- 

 tion of this, corresponding to a brain, pass off nerves in front, among which 

 the two optic nerves are conspicuous for their length ; while its posterior 

 arc, which surrounds the cesophagus, has three transverse commissures suc- 

 ceeding each other. From the lateral portions of this ring, pass off six 

 pairs of large nerves for as many pairs of prehensile feet. From its pos- 

 terior border arises a large trunk composed of two bands which extend 

 backwards along the median line of the abdomen, furnishing nerves to the 

 fin-like and gill-like appendages, and then separating, terminate in the tail 

 in a ganglion from which are given off many filaments to the neighboring 

 parts, and a very long one which enters the caudal spine. '-^* 



Of the nervous system of the Phyllopoda, that of the genus Apus is the 

 best known.'*' The brain consists of a flattened, quadrilateral body, fi'om 

 the superior angles of which arise the optic nerves, while from the posterior 

 angles pass off the two long, oesophageal commissures. These last, before 

 reaching the thoracic ganglia, are connected by a transverse commissure. 

 Upon the thoracic succeed numerous abdominal ganglia, those of each pair 

 of which, as in the first, are wide apart, but they gradually approximate 

 posteriorly, and at last are fused into a single mass. 



The two thoracic ganglia, as well also as the anterior abdominal pairs, 



21 Geophi/us. Verm. Schrift. II. p. 16, Taf. IX., and Newport, 



ai Treviranun, Verm. Schrift. It. p. 31, Taf. Pliil. Trans. 184:i, p. 247, PI. XI. fig. 1. Tlie 



Til. fix. 2, 5 (Lithohiiis and Geophilus) ; Kii- number of gani,'lia increases in general with the 



torga, l')C. oit. p. 15, Tab. II. fit;. 2, Tab. III. fig. increase of the bi)ily togetlier with that of the nunj- 



1, 2 {Scolopendra morsitans) ; but especially ber of the segments and feet. 



Newport, Philos. Trans. 18;U, p. 40S, PI. XVII. 2.5 yan der Hoeven, llecherch. &o. p. 21, PI. 



fig. 4:5-4S (Scolopendra), and 1843, p. 257, PI. XI. III. fig. 2, 3. 



fig. 11-13 (GeophUu/)). ^ This system has been described by Gacde 



23 Newport, loc. cit. 1843, p. 252, PI. XI. fig. (iriedemann\i Zool. Magaz. I. Stück 1, p. 91, Taf. 



0, 10, or Owen, Lect. &c. p. 200, fig. 99. I. fig. 1), and by lierlhold (Isis, 1830, \>. 690, Taf. 



-' Th';re are only six jiairs of ganglia with VII. flg. 4). But for the mist careful researches 



Glomeris ; see Brandt, in Muller^s Arch. 1837, on this subj(;ct we are indebted to Zaddach (loc. cit. 



p. 3Ü, Taf. XII. fig. 6. But these ganglia are p. 35, Tab. III.) 

 very numerous with Julus ; see Treviranus, 



