VERMES. 



fokii, vciiuj, niaftra, cardiiim, &c. They are always 

 fcparatc from each other towards the extremities of long 

 (hells, and generally approximate at the edge on which the 

 hinge is fituated, in order that a very fmall relaxation may 

 produce a large opening on tlie oppofite fide. 



The common oyftcr pofTefTes its iirft pair of valves, con- 

 fifting of fingle laminas, before it leaves the parental organ* ; 

 the mufcle pafTes between the centre of the concavity of 

 each /hell, adhering to each, and it afts on the valves nearly 

 at right angles. The animal has no other continuity with 

 the (hell. As it grows, it augments the margin of its iliells, 

 and thickens them by adding new laminx on the infide ; the 

 mufcular adhefion glides forward, (till keeping to the centre 

 of the valves. 



Many of the teftaceous moUufca have the power of re- 

 moving themfelves from one place to another, by means of 

 a mufcular appendix, which they can protrude or retraft at 

 pleafure, with which they fallen themfelves to the fand and 

 rocks, and thus drag themfelves along. This appendix is 

 called the foot of the animal. 



The common oyfter, the fpondylus or thorny oyfter, 

 fome fpecics of the fcallop, the anomia, and in general all 

 the mollufca that have (hells with unequal valves, iiave no 

 foot, and are, therefore, deprived of the means of voluntary 

 locomotion. 



One of the moll fimple of thefe feet is that of the fre(h- 

 water mufcle (mytilus anatinus, Linn. ; anodontites, Cuv.) 

 It is fituated before the body, towards the margin of the 

 (laells. Its form is a compretTed oblong. We obferve on 

 each fide externally a layer of fibres, proceeding from the 

 bottom of the (hell. There are alfo fome internal fibres, 

 which crofs each other at right angles ; and others unite the 

 two external layers, to which they are attached in a circular 

 manner. From this difpofition it will eafily be underftood, 

 that the animal may, when it pleafes, change the three 

 dimenfions of the foot, or of one of its parts : by this 

 means, it is enabled to place its (hell flat on the ground, and 

 to crawl along like the fnail by the help of its foot. 



The mufcle may be obfervcd to open its (hell, to put 

 forth the foot, and elongate it, to feel about with it. The 

 animal fixes it to fome objeft, and drags the fliell after it. 

 The animal called by the French lavignon, alfo a bivalve, 

 puts forth a broad flat foot, by which it makes its way into 

 the fand or mud. It has two long tubes, which keep up its 

 communication with the furface, for the purpofe of refpira- 

 tion. The holes correfponding to them (hew where the 

 animal is. See Reaumur, " Du Mouvemcnt progrefTif, et 

 de quelques autres Mouvemens de diverfes Efpcccs de Co- 

 quiliages, Orties, & Etoiles de Mer," in the Acad, des 

 Sciences, 1710, with fevcral figures, and detailed explana- 

 tion of the fubjeft, both fo far as concerns the animals juft 

 mentioned, and fome others. 



AVe find this fimple foot in the piddock. Its form is 

 almofl fpherical, and tunicatcd by a flat furface. The part 

 which Linnius has oblervcd in the razor-fhells, and which 

 he has compared to a glans in its prepuce, is the foot, by 

 which the animal buries itfolf in the fand, or rifcs to the 

 furface. In thefe two genera, the foot is protruded at the 

 aperture of the (hell, which is oppofite to that through 

 which the tubes pafs. See Reaumur in the Acad, des 

 Sciences, 1712, with figures. 



The foot of the cardium or cockle is fomewhat complex. 

 It has a triangular appendix, which is capable of inflexion, 

 of feizing with its point tlic glutinous matter, and drawing 

 it out into threads. But the fx)ot of the fea-mufclc (mytilus 

 edulis) is the moil remarkable in its organization. It r£- 

 fembles a fmall tongue, mai-kcd with a longitudinal furrow, 



fufceptible of confiderable elongation, and of being lliortened 

 into the form of a heart. This organ is moved by five 

 mufcles on each fide. Two arifo from the extremities of 

 the (hell, near thofe \yhich clofe it ; the other three come 

 from the bottom of the fliell, and the depreffion for the 

 nates. They are all inferted into the foot, with the fibres 

 of which they are interwoven, in the fame manner as the 

 external mufcles of the human tongue join the lingual. 

 The organ is completely enveloped in a (heath formed of 

 tranfverfe and circular fibres, of an obfcure purple colour. 

 This foot is employed both in fpinning and crawling : the 

 laft; office is performed as in all the other bivalves. It ac- 

 comphflies the firft; by feizing with its point the gluten 

 fupplied by a gland fituated under its bafe, and drawing it 

 out into threads, in the above-mentioned furrow. The 

 gland that fecretes this humour, of which the thread is 

 formed, will be defcribed hereafter. 



The organs of motion in worms are not fo perfeft as in the 

 larvK of infefts ; having neither fcaly nor membranous feet, 

 feveral of them crawl or drag themfelves along by the help 

 of ft.i(r hairs or briftles, with which they are wholly or 

 partly covered : of this defcription are the ger.era aphro- 

 dita, terebella, nereis, lumbricus, &c. Two kinds of mufcles 

 contribute to their motion. 



The one extends the v.hole length of their body, and 

 forms four principal fafciculi, two of which belong to the 

 belly, and two to the back. Thefe four mufcles may be 

 faid to conflitute the mafs of the body. We find them im- 

 mediately under the flciu. Their fibres are parallel ; but- 

 their length does not exceed that of the rings, being inter- 

 rupted in the folds of each ring by a very compaft cellular 

 tilTue. The flrufturc of thefe mufcles is, however, mod 

 dillinftly obferved iu the infide. We there find that they 

 are feparated from each other by a longitudinal line, and 

 enveloped in a kind of fac of a clofe cellular fubftance, 

 which correfponds to each ring of the body. Thefe four 

 mufcles produce the principal motions. Where thofe of the 

 back contraft wholly or partially, they raife the portion of 

 the body to which they belong : the fame cffeft, but in tlie 

 oppofite direilion, is produced by the conilruftion of the 

 ventral mufcles. 



The fecond order of mufcles is appropriated to the mo- 

 tions of the fpines or briilles. Their number is equal to that 

 of the tufts of hairs. The defcription of one of them' 

 will be fufficient to give us a knov«ledge of the whole. 



The hairs, briflles, fpines, &c. which project from the 

 bodies of thefe animals, are manifeftly moveable. They are 

 rctrafted, and puflied out at pleafure. The mufcles which 

 produce thefe motions are vifible only when the animal is 

 laid open, the intellinal canal taken out, and the flcin 

 llripped o(f. We then obferve that each tuft of hair is re- 

 ceived in the concavity of a flc(hy cone, tlic bafe of which 

 is attached to the longitudinal mufcles, and the apex to the 

 internal extremity of the hairs. All the fibres which form 

 this cone are longitudinal, but enveloped by a compaft cellu- 

 lar fubilancc. They move the hairs outwardly, and in the di- 

 reftion which their contrailion may determine. This firll 

 clafs of the mufcles, which belong to each branch of hairs, 

 may be called the protractors of the fpines. 



The fpines arc withdrawn within the body by another fet 

 of mufcles, which may bo called retratlors. They have 

 fewer fibres than the former ; their adion therefore is feeble. 

 They are fituated under the internal furface of the long 

 mulcles, at a fliorl diilance fiom the holes with which the 

 latter arc perforated for the paflage of the hairs. Tliey arc 

 inferted into the tufts of Ipines, nearly on a level with the 

 point, which thcfc reach, when completely rctraClcd. It 



may 



