VERMES. 



ttie fame direSion. When he has reached the furface, the 

 vertical pofition caii have no other eftetl than that of retaining 

 him in the fame pollure and place ; to change it, he mull 

 again incline lii: body. In this way, all the medufjc with 

 gelatinous and orbicular bodies fwim : the umbelia remains 

 parallel to the horizon only in the ftate of red, or at I'-all of 

 relative repofe. Defcending in the water is acc^ir 'iiOied 

 very fimply ; their fubftance being fpecifically heavier than 

 that of fea-water, it is only neccffary that tiiey ftiould con- 

 trail thernfelves powerfully, fo as to coatraft their dimen- 

 fions in every direftion, and they fink of themfelvcs. Some- 

 times, in order to go down more quickly, they turn thern- 

 felves over ; fo that the upper convex part of tlie umbelia is 

 downwards." Amiales du Muieum, tom. xv. p. 41. 



The coriaceous Ikin which covers the aftiniae, poilcfles fo 

 extraordinary a power of contraftion.that thefe animals can 

 aflume at pleafure the moft diffimilar forms. Sometimes they 

 are flattened into a diflc ; fometimes elevated into a cone ; 

 fometimes lengthened into a cylinder, &c. &c. 



" They can walk," fays Reaumur, " in two ways ; firft, 

 by means of their bafis, of which they can change the 

 figure, dilating or contratting it in different direftions, fo as 

 to move forwards the body llowly." Reaumur defcribes 

 this at great length ; Acad, des Sciences, 1760, p. 470, 

 et feq. " I have alfo," fays he, " feen them walk upon 

 their tcntacula. They were the kind that live in holes of 

 rocks, and poffefs long tentacula in proportion to their fize. 

 In this cafe the animal is inverted, the bafis being upwards. 

 The tentacula are very vifcous, and even rough to the 

 touch, fo as to be well calculated for the purpofe." P. 47 j. 

 He has reprefented them in the different forms which they 

 can affume, in fig. 21 — 26. 



In frefh-water polypes (hydra), we obferve moveable ten- 

 tacula about the mouth, which feem principally deftined to 

 feize their prey. The animal has the power of locomotion. 

 The fmallnefs and tranfparency of parts m the other genera 

 do not allow of our difcovering the mechanifm by wliick 

 motion is produced. 



The two following memoirs of Reaumur, in the Academy 

 of Sciences, contain the beft account of the motions of 

 thefe animals, and they are illuflrated by fereral figures. 

 " Du Mouvement progrcffif, et de quelques autres Mouve- 

 mens de diverfes Efpeces de Coquillages, Orties, et Etoiles de 

 Mer," 1 7 10, p. 439 ; " Obfervations fur le Mouvement 

 progreflif de quelques Coquillages de Mer, fur celui des 

 Heriffons de Mer, et fur celui d'une Efp^ce d'Etoile," 

 1712, p. 115. 



Nervous S^em. — Animals without vertebrae are not 

 formed on a common plan, either with refpeft to the nerves 

 or mufcles ; they prefent difparities fo great, and indeed are 

 fo deficient in common charafters, that we are obhged, with- 

 out making any general obfervations, to confider the nervous 

 fyftem in the difiej-ent clafTes and the principal genera. 



Brain and Nerves of the Cephalop'idous Molhifca In tlie 



fepia oilopus, the cutlle-fifli, and the calmar, tlie nervous 

 fyftem appears to refemble in fome rcfpcdts that of red- 

 blooded animals. The brain is inclofcd in a particular 

 cavity of the cartilage of the head, which is pierced by a 

 number of holes to give palTage to the nerves. The carti- 

 lage of the head has the form of a lioUow and irregular 

 ring ; its pofterior part is the thickcft, and containj the 

 brain ; its anterior part contains the ears, and a feniicircular 

 canal which communicates on each fiilc with the cavity of 

 the brain, and includes the medullary collar. The ccfopha- 

 giis pafTes through the centre of this cartilaginous ring, and 

 js confequently, as in all white-blooded animals, furrounded 

 ^y the medullary cord. The lateral parts of the carti- 



VOL. xxxvii. 



laginous ring have eminences which form a kind of orbit on 

 each fide. 



The brain is divided into two diftinft parts ; one next the 

 oefophagus, the furface of which is f?nooth, and tlic other 

 towards the back, which is round, and marked by longitu- 

 dinal ftrise. The medullary collar arifes from the lateral 

 parts of both portions : in the oAopus it is in the form of 

 a lamina, the anterior part of which produces four large 

 nerves, whicii, with the four correfponding nerves, proceed 

 forward into the eight feet, which crown the head. Thefe 

 lamina: arc joined inferiorly, and tluis furround the oefopha- 

 gus. Two other principal pairs of nerves arife on each fide, 

 near the origin of the collar. The firll or optic pair ex- 

 tends diredlly into the orbit, pafTes after a fhort courfe 

 through the fclerotic coat, and is there dilated into a gang- 

 lion larger than the brain, (liaped like a kidney, with the 

 concave fide turned towards the brain. The fubftance of 

 this ganglion appears to be the fame as that of the brain : 

 its convexity produces a multitude of fma'l nerves, as fine as 

 hairs, which pafs through the choroides, by an equal num- 

 ber of fmall holes, to form the retina. The fecond pair 

 belongs to the mufcles of the fac ; it originates a hltle 

 above the preceding pair. Thefe nerves defcend obliquely, 

 and after leaving the cerebral cavity, pafs between the 

 mufcles, which fuftain the head, to the lateral part of the 

 fac, near its fuperior edge, between the body and the bran- 

 chiae. It then divides into two branches, one of which 

 defcends to the bottom of the fac, the other dilates into a 

 roundifh ganglion, which produces a multitude of nerves, 

 difpofed Hke radii. Thefe are diftributed to all the flefity 

 fibres of the fac and the fins. 



The anterior and inferior part of tlie collar gives origin 

 to two pairs of nerves. The firft or auditory are very 

 (hort, as they only traverfe a cartilaginous lamina to pene- 

 trate the ear, where they are diftributed. The fecond pair 

 iffues from the cartilage by two lioies placed near each other, 

 and beneath the ears : the two nerves which compofe it de- 

 fcend within the peritoneum to the bottom of the fac. 

 When they arrive near the heart, they form a complicated 

 plexus, from which all the nerves of the different vifcera 

 proceed. 



Each foot has a nerve, which paffcs from one extremity 

 to another, like an axis, and occupies a canal, w-hich we have 

 defcribed in fpeaking of the mufcles. This nerve is en- 

 larged, at different fpaces, by numerous gangha, which have 

 the appearance of tubercles, and from each of which ten or 

 twelve nervous filaments proceed : thefe diverge and pene- 

 trate the mufcles of the interior of the foot, to which they 

 diftribute branchiae ; but the chief ramifications arc fpe nt 

 on the fuckers. 



This defcription is taken from the oAopus : the other 

 cephalopoda differ only in having a brain lefs diftinftly di- 

 vided, and prefenting lefs confpicuous furrows. 



Nervous Syjlem of the G ajleropodous MoUufca. 



In the Snail {Helix Pemalia.) — The brain ia fituatcd upon 

 the oefophagus, behind an oval mafs of mufcles, which cn- 

 Tilop the mouth and the pharynx. Its (hape is neaily ami- 

 lunar, with the concavity backwards. The angles of the 

 crefcent are prolonged on each fide into a branch, by which 

 the oefophagus is encompafled in a collar. The falivary 

 glands, and the mufcle which retrafts the mouth and brain, 

 pafs alfo through this collar. The two cords produced by 

 the brain unite below the oefophagus and mufcle in a large 

 round ganglion, which is more than one-half the fize of the 

 braip. All the nerves proceed from one or other of thefe 

 two maiTes. Thofe furnifhed by the brain proceed from the 

 D Uicral 



