INSECTS. 



mufcles. The former being made like nicaths, and having 

 their cavities filled by the miifclcs, cannot hs arliculatcd by 

 fimple and plane furfaces ; their joints therefore exhibit no 

 fpccimens of arthrodia and enarthrofis. All their moveable 

 articulations may be reduced to three kinds. Where the 

 parts require a folid point of fiipport, the ginglymus is em- 

 ployed ; the horny coverings of the limbs, being tubular, 

 mull rell on each other in two points of their circumference 

 atleaft, and this arrangement neceffarily determines the gin- 

 glyinoid form. Wiiere a folid point of fupport is not re- 

 quired, the parts are merely fufpended by ligaments, or are 

 articulated by a kind of fetting in. In the latter con- 

 ftruclion one part enters and is received into the hollow of 

 another. Thus the thighs of infefts are fet in the thorax, 

 and the rings of the abdomen are fet into each other. Since 

 the part which receives, and that which is received, are fcg- 

 ments of a fpheroid, the latter may perform a twitting mo- 

 tion : it may be puihed more or lefs iompletclv into the ca- 

 vity, either equally in its whole circumference, or to a 

 greater degree on one or the other f.d-- ; but flexion properly 

 ib called cannot be performed. The parts which are con- 

 nected by ginglymus, and which are chiefly the different 

 portions of their legs, have deep notches on tliat fide where 

 the flexion is moft extenfive ; the interval is filled with a 

 pliant membrane, and there is no other ligament. The arti- 

 cular tubercles and excavarions are fo arranged that they 

 eannot be luxated without fra<fture. 



Mtifcks — The iSufcular fyftem of the cruftacea is con- 

 fined to the motions of the legs, tail, and falfe feet ; there 

 are no mufcles for moving the head on the thorax, fince tlie 

 two parts are confolidated into one piece in this order. The 

 antenna, the jaws, and the palpi, have particular mufcles, 

 but we rtiall not confiderthem at prefent. The tail is a ve.-y 

 principal part of the body in moil crullacea. It confifts of 

 a very ftrong and moveable member, wliich they employ with 

 great effeft both in jumping and fwimming. It is compofed 

 in feveral monoculi of long threads, which in the poly- 

 phemus (molufcacrab) are folid and moveable at their bafes 

 only. In the crabs it is (hort, flattened, and curved under 

 the body in a depreflion placed between the legs. The can- 

 cer bernhardus (hermit crab) has a foft tail without fcales, 

 which it introduces into empty {hells or hollow fl;ones. 



The tail deferves a particular defcription in the lobfter, 

 where it is formed of fix chief fegment.-:, and terminated by 

 live plates. The former are not all of exaftly the fame 

 figure; they are convex above, and cover one another like 

 tiles ; below they are nanower, and united by a locfe mem- 

 brane which allows them confijerable mobility. In the 

 latter fituation, at the angle of union between their dorfal 

 and lower portions, they liave a kind of firm fin?, bordered 

 \<nth ciliated proceffes, and formed of feveral articulations. 

 Thefe are called talfe or fwimming legs (pedes natatorii). 

 Thcfe admit of being moved from before backwards, and a 

 little from without inwards, by means of fmall mufcles con- 

 tained in their interior, but not differing fufficiendy from 

 thofe of the true legs to merit a particular defcription. 



The five plates, which terminate the tail, confiil of two 

 pairs and a finglc one ; the latter is placed in the middle, and 

 articulated dirtdly to the lad fegment. The opening of the 

 anus is found under it. In fome fpecies it is divided in its 

 middle, fo as to admit a flight motion. Each of the lateral 

 pairs is fupported by a common piece articiJated to the laft 

 fegment of the tail. The internal plate of each pair is fim- 

 ple, and ciliated, like the middle one, only at its extremity ; 

 but the external is in a manner articulated, or rather formed 

 of two pieces ; the firft of thefe covers by its extremity, 

 which is denticulated, the fmaller wLicfa fucceeds, and of 



which the edge is fet with very clofe cilia. The nmfcles 

 which move this tail are fo Cngular, as to merit a dillinft 

 defcription. 



The nr.ifeles of the ia'il, in the lohjler, form two mafPes, 

 feparated from each other by the intellinal canal. In tl,e 

 dorfal mafs, which is the thinneit, and Icaft complicated, 

 three kinds of fibres may be obferved. The firft conftitute 

 a mufcle attached to the dorfal portion of the thorax towards 

 its pofterior fourth part. It is then direfted obliquely 

 backwards and outwards towards the fides of the firft feg- 

 ment of the tail, in which it is inferted. The portion of one 

 fide adling feparately, will carry the tail towards the right or 

 left ; when both aft together, they will extend the tail, and 

 maintain it in the extended ftate. The fecond and third 

 ferics of mufcular fibres are extended through the whole 

 length of the tail in two parallel and contiguous lines. They 

 ariie from the upper and lateral portions of that feptum of 

 the thorax, on which the branchis are applied, by feveral 

 digitations. When it has arrived at the firft ring of the tail, 

 a fmall interfedtion is obferved on the fiirface, and we ob- 

 (erve that a fmall bundle of fibres turns off to be inferted 

 into this firft annular fegment ; and a fimilar infertion takes 

 place in each of the fticceeding rings. This difpofition 

 gives to the internal band the appearance of a twifted cord. 

 The externa! portion of the dorfal mafs is formed of diftinft 

 longitudinal fibres. Thefe three orders of mufcles have 

 much analogy to the ftraight dorfil mufcles of caterpillars, 

 as we {hall fee in the fequel. 



The veiUra! mafs of the mufcles of the tail is much thicker 

 and more complicated than thofe of the back. In order to 

 give a clear notion of its compofition, we fliall defcribe it as- 

 obferved in three afpefts. In the firfl place, from the back ; 

 the mufcles which we have defcribed, as well as the inteftinal 

 canal, having been removed ; fecondly, from below, the 

 fcales which cover the under furface of the tail, and the 

 nerves having been taken away ; and thirdly, on the infide, 

 a longitudinal feftion having been carried through the mufcle 

 in its middle line, in order toexpofe the internal ftrufture. 



The ventral mufcle of the tail, on its dorfal afped, arifes 

 in the interior of the thorax, above the intricate bony texture, 

 containing the mufcles of the legs. It is then divided into 

 a right and left portion ; each of which is formed of three 

 broad digitations. Over the firft fegment of the abdomen, 

 its longitudinal fibres dip under others, which turn round 

 and embrace them. The reft of the mufcle, in the whole 

 length of the tail, is thus formed of two feries of convex 

 fibres curved in parallel dircdlions, placed laterally with re- 

 gard to each other, and feparated from right to left by a 

 channel in v.'hich the inteftinal canal is lodged. 



On the under furface three very diftind orders of fibres 

 may be feen. The firft is produced from the inferior furface 

 of the digitations which arife from the bony frame-work of 

 the thorax. The fecond confifts of obhque fibres, which 

 are a continuation of the former, extending from the m.iddle 

 line, in which the nervous cord is placed, to the fides of the 

 rings at the angle of union between their dorfal and ventral 

 portions. Each of thefe angles, from the firft to the fixth, 

 has two ftrong bundles. The third feries is made up of 

 tranfverfe bundles delcnbing arcs, of which the convexity is 

 downwards. Thefe mufcular flattened hoops correfpond 

 to the interfeftions of the rings, and feem to form derivative 

 pullies for the oblique fibres juil mentioned. 



Laftly, the ventral mufcle of the tail, divided longitudi- 

 nally in its middle, refemblcs a rope with the ftrands rather ' 

 obliquely difpofed. The fibres belonging to the tranfverfe 

 bundles are dittindl and narrower. 



From tiiis Angular complication it rcfults, that the mufcle, 



when 



