INSECTS. 



The other orders of infers have nearly the lame conforma- 

 tion as the coleoptera. 



The mufclcs of the tibia are fituated within the femur. 

 The extenfor, which is lliort and (lender, is attached to its 

 external edge (the femur being fuppofed extended in the 

 longitudinal direction of the body) : the flexor is much 

 flronger and longer. It is fituated on tlie inner fide, and in 

 the whole of the fnperior part. 



There are likewife two innfcles to each pint of the tarfus : 

 one, which is fmall, and placed on the fnperior or dorfal fur- 

 face, ads as anextenfor. The otl-.er, which is more confpi- 

 cuous, and fituated on the inferior fide, afts as a dcxor. 



For the dcfcription of the wings, fee Entomoi.ogv. 



Tlie muftlcs that move the wings are not yet well afcer- 

 tained. They appear to be two kinds. Some, which are 

 fmall and ihoit, are fitted to extend or fold the wings, at 

 the fame time that they move them to or from the body : 

 "the others; which are fomewhat longer, are calculated to 

 produce the motions of elevation and depreflion which the 

 wings perform. 



The elytra of the coleoptera, orthoptera, and liemip- 

 tera, do not appear to afliil in the adion of flying, at Icall 

 they are not moved in the fame manner by the mufcles of the 

 peftus. 



Organs of Saifalion. — On comparing together all the ner- 

 vous fyftem,5, wo find only one common part, wliich is a An- 

 gle tubercle, fituated at the anterior extremity of the fyllem, 

 and always producing two lateral tranfverfe fafciculi or crura, 

 which unite it to the rc.l of the fyfteni. This part appears 

 always to correfpond to that named cerebellum in man. The 

 cerebellum of animals that have red blood and vertebra-, is 

 always preceded by fevcral pairs of tubercles, fornjing 

 tifually a larger mafs than ihe cerebellum itfelf. and united 

 to t!ie reft of the fyftem by two longitudmal fafciculi or 

 crura, which are interwoven in croffing with thofe of the ce- 

 rebellum. Tliib union takes place in fuch a manner that both 

 are confounded in the common mafs which forms the root of 

 the medulla oblongata, atid medulla fpinalis, and leaves ra 

 fpace between tliem. Thefe tubercles make what we call the 

 cerebrum. 



In the white-blooded animals, or thofe that have no verte- 

 brae, there are aUb tubercles correfponding to the cerebellum, 

 but they are much fnialler, removed from each ot'.ier, and 

 connecled with the ceiebellum only by frparate nervous fila- 

 tnents. The crura of the cerebellum leave a large interval 

 between them, which receives the osfophagus as in a 

 collar. 



The two fafciculi, forming. the medulla fpinahs, are inti- 

 mately united in vertebral animals, and no trace of their re- 

 paration remains, except a longitudinal furrow before and 

 behind. But, in the animals which have no vertebra, when 

 this produclion exifts, it is formed below the cefupliagus by 

 the union of the two crura of the cerebellum. Thefe two 

 fafcicuU commonly remain diilinft throughout tlie greater 

 part of their length, and are only united at different fpaces 

 by knots from which the nerves proceed. This production, 

 however, frequently does not exill. In fome animals with 

 •white blood, which have no elongation of the medullary 

 fuliftance, as in the mollufca, the nervous trunks, which are 

 derived from the crura of the cerebellum, enlarge and form 

 ganglia, or two or three nerves unite with each other to 

 produce a common ganglion ; and in general it is only from 

 their ganglia that the nerves which are diftributed througl- 

 cut the body take their origin. 



In thofe white-blooded animals that have a knotted and 

 double medullary production, that is to fay, in the infeCls, 

 cruilacea, and certain worms, the iiervci- all arife from the 



knots or ganglia of the medulla, or from fome of the ante- 

 rior ganglia of the cerebellum. 



Ill tlic red-blooded animals the nerves of the fpine arife 

 from the medulla fpinahs in two bundles of medullary fila- 

 ments, which unite after the pollerior bundle has formed a 

 ganglion. They afterwards feparate into two trunks, the 

 anterior of which communicates with the great fympathetic 

 nerve by one or two filaments, and a ganglion is always 

 formed at the place where they unite. 



The great fympathetic nerve, which is conftantly found 

 in all red-blooded animals, exiils in none of thofe with white 

 blood ; unlefs we fljould regard as that nerve, the two 

 nervous cord? which unite all the ganglia, and jvhich are 

 commonly called medulla fpinalis in the craltacea, inlefts 

 and worms. Were this opinion adopted, a medulla fpinalis 

 would be no longer attributed to thofe animals, and the ab- 

 fence of that production would then be the common charac- 

 ter of all the white-blooded clafies. 



Inft ead of having the parts, which rcprefent the brain an J 

 medulla fpinahs enclofcd in bony cavities, the lower orders of 

 animals have them inclofed in the common cavities, with the 

 other vifccra. 



Brain and Nerves of the CruffacM.— The cruftacca, which, 

 in their organs of motion, very much refemble the other 

 infefts, though thofe of circulation and refpira'i a are ex- 

 ceedingly different, have alio a fimilar nervous fyilem, at 

 leitl in the eflential parts.- 



In fuch of the genus cancer as- have long tails, tlie middle 

 part of the fyllem is a knotted cord, which extends from 

 one citrcmity of the body to the other. The fliort-tailed 

 kind, commonly called cnil>s, have a medullary circle in the 

 middle of the abdomen, whence the nerves of the body 

 proceed like radii. 



In thefe animals the brain is placed at the anterior extre-' 

 mity of the fnout, and confequently at a confiderable dif- 

 tance from the mouth, which opens under the corfelet. O.T 

 this account the cords which n.ake the collar of the oelbpha- 

 gus are more elongated than in other animals. 



A. Brain of the common Cray-Fyh, {yljlacus F'wvlalility 

 Fab.) — The brain of this animal forms a mafs which is 

 broader than long, and ditlintlly divided on the fuperior 

 fiirface into four round lobes. Each ot the middle lobes 

 produces an optic nerve from its fore part. This nerve pro- 

 ceeds directly into the moveable tubercle which fultains the 

 eye, and is there dilated and divided into a multitude of fila- 

 ments, which form a pencil, and unke to all the fmall tu- 

 bercles of the eye. 



Four other nerves arife from the inferior fiirfacc of the 

 brain ; thefe proceed to the fiur anteiu!;e, and detach fome 

 filaments to the neighbouring parts. The cords which form . 

 the collar, arife from the pollerior part of the brain. About 

 the middle of its length, each detaches a large nerve, which ■ 

 extends to the mandibles and their mufcles,. Tiefe cords 

 unite under the ftomach in an oblong ganglion, which fur- 

 niflies nerves to the different pairs of jaws. On leaving this 

 part, the two cords remain near e.ach other througliout the 

 whole length of the corfelel, whire they form five fucceffive 

 ganglia, placed between, the articulations of the five pairs of 

 feet : each foot receives a nerve from its correfponding gan- 

 glion, which penetrates to the extremity of the foot : the 

 nerve of tlie forceps is the largelt. The medullary cords ■ 

 extend into the tail, where they are fo intimately united that 

 it is not poflible to dulinguifh them. 1' hey form fix gan- 

 glia, the five firft of which produce each two pairs of 

 nerves ; the lall produces four, which are diftributed as 

 radii to the fcaly fins that terminate the tail. 



The hermit crab, {Paguriu, Fjibr. Bernhardus, Linn.) 



the 



