VERMES. 



the formation of the ganglion, which furni(hes nerves to the 

 vifcera, the two trunks penetrate by two different lioles into 

 the fubftance of the mufcle of the foot. Tlicfe two holes 

 arc the origin of two canals, which run throughout the 

 vrbole length of the foot, on the fides of another middle 

 canal, which appears deftined to diftribute the blood of the 

 animal. The two nerves, lodged in the lateral canal, are 

 diftributed by a great number of lateral holes into the fub- 

 ftance of the fieihy mufcles of the foot, and of the fhell, 

 where they may be foUowed with facility. 



In the Helix Stagnalis and Helix Cornea ( Planorbis Cornea], 

 the brain confills of two lateral mafTes, feparated by a 

 contraclion. In the living animal they are of a lively red 

 colour. The diftribution of the nerves differs very little 

 fr«m what we obferve in the common fnail. 



Nervous Syjlem of the Acephalous Mollufca — It is formed 

 on a plan far more uniform than that of the gafteropoda. 

 In all the teftaceous acephala, from the oyfter to the 

 pholas, and the teredo, there appears no effential difference : 

 it confifts always of two ganglia, one on the mouth re- 

 prefenting the brain, and another towards the oppofite part. 

 Thefe two ganglia are united by two long nervous cords, 

 which take the place of the ufual collar, but which occupy 

 a much greater fpace at the foot where it exifts, and the 

 ftomach and Hver always pafs in the interval between them. 

 All the nerves arife from the two gangha. 



In the AnoJontites , or Frejh-iuater Mufcles, in Cockles, in 

 the Venus, Maclra, and Mya. — In thefe, and generally in 

 all the bivalves which have two cylindrical mufcles, one at 

 each extremity of their valves, for the purpofe of bringing 

 them together ; the mouth is placed near one of thofe 

 mufcles, and the anus near the other. The foot appears 

 about the middle of the Ihell ; and the tubes for the excre- 

 ments and refpiration, when they exift, go out at the end 

 of the {hell oppofite to that in which the mouth is fituated. 

 The brain is placed at the anterior edge of the mouth ; it is 

 oblong tranfverfely ; it fends off two cords anteriorly, which 

 go to the adjacent mufcles, and turning towards each fide, 

 penetrate the lobes of the cloak, paffmg through the whole 

 extent of their edge. The brain furnilhcs alfo, on each 

 fide, fome filaments to the membranous tentacula, which 

 furround the mouth, and detaches, from its pofterior edge, 

 the two cords analogous to the medullary collar in other in- 

 vertebral animals. Thefe cords proceed, each on its fide, 

 under the mufcular itratum which envelopes the liver and 

 the other vifcera, and which becomes thicker as it is con- 

 tinued to form the foot, which is frequently conilrufted for 

 fpinning. When arrived at the polterior mufcle whicli 

 clofes the valves, thefe cords approach each otlier, and en- 

 large as they unite to form the fecond ganglion. This 

 ganglion has the form of two lobes. It is at leall as large 

 as the brain ganglion, and always much more eafily diftin- 

 guifhed. It detaches two principal nerves on each fide, 

 and the four together reprefent a kind of crofs. The two 

 anterior nerves, as they afcend, proceed a little towards the 

 fide of the mouth, and after having defcribed an arc, pene- 

 trate into t!ie branchix. The other two pafs on the pof- 

 terior mufcle, prccifely in the fame manner as tliofc of the 

 brain on the anterior. After detaching fome filaments they 

 proceed into the cloak, the edge of which they follow until 

 they join thofe of the brain ; they thus form a continued 

 circle. We do not yet know the origin of the vifccral 

 nerves in thefe animals. 



The teftaceous acephala, in which the foot is protruded 

 by an extremity of the fhell, tiiat always remains open, and 

 the tubes by the oppofite extremities, that is to fay, in 

 razor-fifh and piddocks, the mouth, and confeqiiently the 



brain, is always near one extremity. The nerves which pro- 

 ceed from the brain, take therefore a longer courfe before 

 they diverge to join the cloak. The cords of the collar, 

 however, have a much Ihorter diftance to pafs before they 

 unite. There is a confiderable fpace, particularly in the 

 razor-fiffi, between the mafs of the vifcera fituated in the 

 bafe of the foot, and the pofterior mufcle. The fecond 

 ganglion is fituated in the middle of this fpace, between the 

 branchias of each fide : it k round, and much more diftinft 

 than in the other fpecies ; the nerves it produces are how- 

 ever exaftly fimilar. 



In the oyfter, which has no mouth at the anterior part, 

 the brain and mouth are fituated under the kind of hood 

 which the cloak forms towards the hinge. The nerves go 

 direftly into the cloak itfelf. The ganglion is fituated on 

 the anterior furface of the fingle mufcle, immediately behind 

 the mafs of vifcera. The nerves it produces are the fame as 

 in the preceding genera. 



In the Afcidia. — Thefe fmall marine animals are enveloped 

 in an immoveable coriaceous or gelatinous cafe, which' has 

 two apertures ; one for the expulfion of the excrement, the 

 other for the admifiion of water to the branchiae. The 

 branchiae are in the form of a large fac, and are inrlofed, as 

 well as the other vifcera, in another membranous bag, of the 

 fame form as the external cafe, but fmaller, and completely 

 adhering to that cafe at the two apertures only. The in- 

 ferior ganghon is fituated on this membranous fac ; its pofi- 

 tion is between the two apertures, but neareft that which 

 correfponds to the anus ; it produces four principal nerves : 

 two afcend tovcards the fuperior or refpiring aperture, the 

 other two defcend towards that of the excrements. There 

 are fmaller nerves difperfed through all the membranous fac. 

 We have not yet difcovered thofe produced by the brain, 

 nor the brain itfelf, which is doubtlefs fituated as ufual on 

 the mouth. The mouth is in the bottom of the branchial 

 fac. 



In the Tritons of Linnaus, •which inhabit the anatiferous and 

 halanite Shells, [Lepas, Linn.) — Thefe animals approach 

 perhaps nearer to the crultacea, and particularly to the mo- 

 noculi, than to the mollufca. Their nervous fyftem is a fort 

 of middle kind between that of the mollufca, and that of 

 the cruftacea and infefts. 



The brain is placed acrofs the mouth, which is itfelf 

 fituated in the pai't of the body correfponding to the liga- 

 ment, and at the bottom of the fhell. It produces four 

 nerves to the mufcles fituated in that place, and to the 

 ilomach, and two others which embrace the oefophagus, and 

 proceed into that elongated portion of the body which bears 

 the numerous articulated and ciliated horny tentacula 

 which the animal protrudes from its Ihell. Thefe two fila- 

 ments approach, and form a ganglion, and then proceed dofe 

 to each other among thefe tentacula, furnifliing a corre- 

 fponding pair of nerves for each pair of tentacula ; but there 

 are no apparent ganglia at the origin of thefe nerves. 



The general rcfult from the preceding ftatcments is, tliat 

 the nervous fyftem of the mollufca confiils in a brain placed 

 on the afophagus, and in a variable number of ganglia, 

 fomctimes approximated to the brain, and fometimes dif- 

 perfed in the different cavities, or placed under tiie mufcu- 

 lar envelopes of the body : that the ganglia are always 

 conneAed to the brain and to each other by nervous cords, 

 which eilablifli a general communication between thefe dif- 

 ferent medullary maffes : that the nerves all aiife eitlier 

 from the brain or the ganglia : and lallly, that there is no 

 part which can be compared to the medulla oblongata and 

 medulla fpinalis. 



Nervous Syjlem of IVormt. — Some genera prcfent a very 

 D 2 diftinft 



