VERMES. 



diftinft nervous fyftem, organized ne:irly like tliat of the 

 cruftacea and infefts. In others, however, that fyftem be- 

 comes fo obfcure, that we can fcarcety recognife its cxift- 

 ence. Thus the clafs of worms, which in feveral of its 

 genera ranks above infefls, with refpeft to the organs of 

 circulation, is reduced almoft to a level with the zoophytes, 

 when confidered with regard to the organs of fenfation. 



The Aphrodite acuhata has a very dillinft nervous fyftem. 

 Immediately behind the tentacula, fituated above the mouth, 

 we obfcrve a large nervous ganglion, which is the brain ; it 

 has the form of a heart, the broadeft and bilobed part of 

 which is direfted backwards. The pointed anterior portion 

 produces two fmall filaments for the tentacula, and the 

 lateral parts forae other filaments, which are ftill more 

 flender, for the parietes of the mouth. This ganglion is 

 fituated immediately above the origin of the cefophagus. 

 The two cords which arife from the brain, and from the 

 collar, are very long and dehcate ; they gradually increafe 

 ill thicknefs as they approach the point of their union. 

 Each then produces a large filament, wliicli we (hall call the 

 recurrent nerve ; thefe nerves are very diftinA : they are 

 direfted forward towards the part where the cefophagus, 

 which is very fhort, joins the (lomach. They may beeaCly 

 followed by the naked eye to the lateral parts of that vilcus, 

 which is very long and mufcular ; before they reach the 

 iuteftines that follow the ftomach, they fwell into a ganghon, 

 which produces a great number of nervous fibrils. 



The two curves of the collar produce a very large 

 ganglion at their union ; it is bifurcated anteriorly, and 

 fituated immediately behind the mouth, and above the cefo- 

 phagus ; it is the anterior extremity of the chief nervous 

 cord. We do not obferve any filaments proceeding from it. 

 To this firft ganglion another fucceeds, which is dillinguiftied 

 from it only by a fmall contraftion ; the latter produces 

 two nervous filaments, which go forwards into the mufcles 

 of the abdomen. A fcries of ganglia, the fpaccs between 

 which are confiderably greater, afterwards fuccced ; each- of 

 thefe fends oft' fix nerves, tliree on a fide, which are loft iii 

 the mufcles. Thefe ganglia are twelve in number. The 

 nervous cord, which fucceeds, and which occupies the pofterior 

 tliird of the body, no longer cxliibits any apparent enlarge- 

 ment ; but pairs of nerves are ftiU detached at certain 

 fpaces. Finally, this cord may be followed to the extre- 

 mity of the body. 



In the Lcich, the nervous fyftem is a longitudinal cord, 

 compofed of twenty-three gangha. The firft. is fituated 

 above tlie cefophagus ; it is fmall and rounded ; anteriorly it 

 produces two flender filaments, which proceed above the 

 difk of the mouth. The lateral parts furnifti a tliick pair of 

 nerves, that form a collar round the cefophagus, as they pro- 

 ceed downward, and unite at the fecond ganglion. This 

 ganglion is of a triangular figure, and appears to be formed 

 by the union of two tuberclfes. Two of thefe angles are 

 anterior and lateral ; they receive the nerves that proceed 

 from the firft ganglion. The other is pofterior ; it is pro- 

 longed into a nerve rather more than half a line long, which 

 produces, the third ganglion : the anterior part of the trian- 

 gular ganglion which we defcribe, detaches two fmall nerves 

 that are loft on the cefophagus, around the mouth. The 

 nine fucceeding ganglia are precifely of the fame form, and 

 produce each two pair of nerves ; they differ only in the 

 greater or lefs diftance at which they are placed froni each 

 other. The third, as we have obferved, is very near the 

 fecondt The three following are at the diftance of nearly 

 a line and a half : but thofe which fuccced, from the feventh 

 to the twentieth, are at the diftance of three or four lines : 

 finally, the, three lall are very clofe together. 



All thefe ganglia are fituated longitudinally below the 

 jnteftinal canal, to which they furnifti, from their fuperior 

 furface, a number of nervous filaments ; they produce on 

 each fide two nerves, which pafs into the longitudinal and 

 tranfverf>; mufcles, in the fubftance of which they are 

 loft. Thefe nerves run in oppofite direftions, fo that 

 they reprefent the figure of an X. The coat of thefe 

 nerves is black, and very folid, fo that before the parts have 

 been immerfed in alcohol, they appear like a fyftem of 

 veffels. 



The nervous cord of the Earth-worm derives its origin 

 from a ganglion fituated above the cefophagus : this ganglion 

 is formed of two clofe, but very diftinft tubercles. It pro- 

 duces a pair of fmall nerves proceeding to the parietes of 

 the mouth, and two large cords, which embrace the cefopha- 

 gus in the form of a collar : thefe unite to form the nervous 

 cord, the origin of which therefore appears bifurcated. 

 Three pair of fmall nerves are detached at this place : one 

 from the cord itfelf, and the others from its lateral parts. 

 They all proceed into the mufcles of the mouth. The 

 nervous trunk is continued to the anus, along the inferior 

 part of the inteftine ; its fize is not fenfibly diminiftied, and 

 the contraftions are not very remarkable : there arc, there- 

 fore, no real gangha. A pair of nerves arifes between each 

 of the rings of the body ; thefe nerves pafs under the longi- 

 tudinal mufcles, and difappear between them and the fl<iii. 

 When the nervous cord reaches the anus, it terminates by 

 forming a plexus, which is loft on the parietes of that 

 aperture. 



In the Gordiiis argill/iccus, there is only a fingle nervous 

 cord, fimilar to that of the earth-worm, but its contraftions 

 are ftill lefs apparent. 



The Nereis and TerebeUa have, within the (kin of the belly, 

 a longitudinal cord, which may be regarded as nervous : it 

 has as many contraftions as there are rings in the body. 

 No nervous filament has been obferved proceeding from this 

 cord. 



In the Sea-ivorm [Lumhrii-iu Marinus, Liiiii.), which in 

 its external charafters approaches nearer to the nereis than 

 to the lumbricus, the nervous fyftem is the fame as in the 

 nereids, but the cord gradually increafes in tliicknefs 

 towards the middle of the body, where it is much more 

 diftinft. 



In the Afcaris LumbricoiJes of Man and the Horj'e. — This 

 animal appears to have two nervous cords ; they are ob- 

 fervable tliroughout the whole length of the body, on the 

 lateral parts of the abdomen. They unite above the cefo- 

 phagus, exaftly at its origin on the mouth ; they are very 

 (lender, and produce no remarkable ganghon : they are fmaller 

 at their origin than towards their extremity, that is to fay, 

 towards the anus ; but they are equal, and precifely fimilar 

 to each other with refpeft to their different parts. We ob- 

 ferve at firft fome fmall granular points, which enlarge in 

 proportion as the nerve defcends. When it has reached the 

 middle of the body longitudinaOy, it forms fquare gangha, 

 at a (hort diftance from each other. Laftly, towards the 

 termination, for the length of nearly fix fines, the nerve be- 

 comes more and more flender, and ends in a very fmall fila- 

 ment, which unites with that of the other fide. 



Thus we find an evident analogy in the organization of 

 the nervous fyftem of cruftacea, inlefts, and worms, no lefs 

 rtriking than that which prevails in the external forms, in 

 the difpofition of the mufcles, and the fingular divifion, into 

 a feries of rings or fegments, which we obferve in thefe 

 animals. This analogy prevents us from eftabhfhing be- 

 tween thefe three cla(les limits equally diftinft with thofe 

 which fubfift between them and the mollufca. The uniform 

 6 diftribution 



