VERMES. 



lateral parts of its.convex fide. There are, firft, two nerves 

 for the flefhy part of the mouth ; next, one on each fide 

 for the fmall horns ; then two for each great horn, one of 

 which proceeds to the bafe of that horn, and paffes into its 

 tnufcular fubftance ; the other goes to the eye. The latter 

 IS folded confiderably on ilfeff, when the horn ie drawn 

 inward. There are befides fome other filaments, which ex- 

 tend to the bafe of the parts of generation, and to the 

 mufcles which move the head. The large inferior ganglion 

 produces at iirft three great nerves, one for the penis, ano- 

 ther for the brain, and a third for the mufcles, which draw 

 the whole animal into its (liell. The inferior furface of this 

 ganglion afterwards produces two great fafciculi, which 

 proceed backward, and which, after paffing between the 

 two mufcles before mentioned, are diftributed to all the 

 flefhy parts of the foot. 



Swammerdam's figure of the nerves of the fnail appears 

 to have been taken from the (lug. 



In the. Slug [Limax Rufus.) — The brain is alfo fituatcd 

 behind the cefophagus in this animal, but it has the form of 

 a narrow ribbon lying crofswife. It enlarges a little at its 

 lateral parts, each of which produces a filament to encircle 

 the cefophagus. The ganglion, wliich is formed by the 

 union of thefe two filaments, is larger than the brain. 

 ' Two principal trunks proceed, each on its refpeftive fide, 

 in a ftraight line from this ganglion. They extend along 

 the lower part of the body, throughout its whole length, 

 preferving nearh' a parallel direction. On the external fide 

 they each detach a number of filaments, which penetrate 

 into the flefhy fubftance of the fkiii. A great number of 

 other filaments alfo proceed immediately from the inferior 

 ganglion to the /kin. Further, the inferior ganglion fends 

 off two nerves on each fide, which go to the vifcera, and 

 •follow the diftribution of the arteries. 



With refpeft to the brain, properly fo called, it furnifhes 

 in the firft place a nerve on each fide for the fleftiy mafs of 

 the mouth ; theii two for each of the great horns, one of 

 which extends to the eye, and becomes the optic nerve. 

 The nerves of the fmall horns arife more outwardly. 



In the Aplvjia This is a fmall marine animal, very like 



the Aug, but refpiring by means of branchias, which form 

 a kind of tuft on the back, and are covered by a particular 

 operculum. The brain is fituated as in the fnail ; but the 

 branches, which furround the cefophagus, produce two 

 ganglia, one on each fide, which are conjoined by a fmall 

 filament. 



The brain fumifties, at its anterior part, two flender fila- 

 ments, which encircle the flefhy mafs of the mouth, and 

 unite under it in a fmall ganghon, whence the nerves of the 

 lips are detached. The brain afterwards affords nerves to 

 the horns and the eyes, which are in this animal fituated be- 

 tween the horns, and to the parts of generation. The two 

 lateral ganglia tranfmit a multitude of nerves to all the 

 flefhy parts of tlie foot and Hiin ; they alfo produce each a 

 long cord, which unites to its correfponding cord on the 

 aorta, near the part where it arifes from the heart ; there 

 they form a ganglion, from which all the nerves of the 

 vifcera proceed. 



In the Clio Boreal'u. — This fmall animal has no foot, and 

 can only fwim. It refpires by two branchix, in the form 

 of wings, fituated on the neck ; but in other refpefts it 

 very much refembles the flug. Its nervous fyftem is ana- 

 logous to that of the aplyfia. 



Its brain is formed of two roundifh lobes : it furnifhes 

 immediately nerves to the tentactJa, and gives origin to a 

 double collar. The anterior extends, as in the aplyfia, 

 under the mouth, to form a fmall ganglion. The pofterior 



has a ganglion on each fide, which furnifhes nerves to the 

 mufcular fliin that furrounds the body ; each of thefe pro- 

 duces one or two other gangha, which fend nerves to the 

 vifcera. 



In the Doris — This is alfo a fmall marine animal fimilar 

 to the flug, but it refpires by external branchix difpofed 

 like ftars round the anus. The brain is very large in pro- 

 portion to the reft of the body, and particularly in com- 

 parifon with that of other galleropoda. It is elongated 

 tranfverfcly, and of a fquare form. It is fituated imme- 

 diately above the origin of the cefophagus, behind the or- 

 bicular mafs of mufcles, which form the parietes of the 

 mouth. 



Six nerves proceed from the brain on each fide j one pair 

 is deftined for the mufcles of the mouth, another for the 

 tentacula. The third is a cord, which pafTes below the 

 cefophagus, and is loft in the mufcles of the foot, where it 

 may be very diftinftly obferved on the lateral parts of the 

 internal furface. The fourth and the fifth are direfted 

 above the mafs of inteftines, and proceed to the fkin of the 

 back. Lailly, the fixth terminates in the parts of generation. 



In the ScylUa This is another marine animal fimilar to 



the (lug, but refpiring by branchije in the form of wings 

 arranged by pairs on the back : it crawls on a furrow in its 

 belly. The collar furrounding the cefophagus is a fimple 

 cord, and does not enlarge into a ganghon as it proceeds 

 downward. The brain, which is above it, is of an oval 

 form ; it fends nerves to the mouth and to the horns, but 

 there are no optic nerves, as this animal has no eyes. The 

 nerves of the vifcera arife from the inferior part of the col- 

 lar, and thofe of the mufcles from its fides. 



In the Sea-Ear [Halyotis Tuberculata.) — This animal has 

 no ganglion above the cefophagus to fupply the place of the 

 brain. We find merely a nervous filament, fituated tranf- 

 verfcly above the cefophagus, behind the mouth. Four 

 fmall ramifications proceed from the middle and anterior 

 part of this filament, two on each fide, and are loft in the 

 parietes of the mouth. At each extremity of the tranfverfe 

 nervous filament there is a very large flat ganglion, from the 

 circumference of which a number of nerves are detached to 

 the adjacent parts. Three filaments pafs off on each fide 

 from the external furface of this ganglion : one is fent to 

 the fetiform tentaculum, fituated above the mouth, the 

 other two proceed to the flat tentaculum, like a buckler, 

 placed more pofteriorly and on the fides. The mofl pof- 

 terior appears to be intended for the eye : it is the thickeft, 

 the other feenis loft in the mufcular parts. 



A very remarkable filament is detached from the fuperior 

 parts : it proceeds above the cefophagus, and joins the cor- 

 refponding one on the other fide. There is a fmall enlarge- 

 ment at the point of union, from which four nerves proceed, 

 two on each fide of the middle line. The moft external is 

 loft in the mufcles of the tongue ; the other purfues the 

 middle line of the cefophagus, and is ramified over the in- 

 teftines. Several fmall branches are detached inferiorly, and 

 terminate in the fan-like mufcles that fuftain the tongue. 



Laftly, the ganglion is prolonged poiteriorly into a thick 

 nervous cord, fituated on the fides and below the cefophagus, 

 which becomes flat, as it proceeds backward : it defcribes a 

 femilonar curve, fo that the two nerves of the oppofite fidea 

 are approximated, and finally touch each other at the bafis 

 of the tongue, and below the anterior part of the large 

 mufcle which attaches the animal to its fhell. The union of 

 thefe two nerves produces a ganglion, from which tn-o very 

 remarkable trunks, intended for the inteftines, proceed ; 

 they can be followed to above the ftomach, and we con per- 

 ceive that fome of their ramifications enter the Uver. Aft^ 



the 



