VERMES. 



diftribulion of nearly equal ganglia upon a cord, extending 

 throughout the whole length of the body, feems defigned 

 to furnifh each fegment witli a brain peculiar to itfelf. 

 Thus we are gradually condufted to that general diffufion of 

 the medullary fubftance, wliich feems to take place in zoo- 

 phytes. 



Animals in 'which no djJlinB NeriMus Syjlem has been yet dif- 

 covered. — We do not, fiiys Cuvier, include in this divifion 

 the animals of the clafs of worms, or the moUufca, in which 

 the minutenefs or foftnefs of the parts have not yet per- 

 mitted us to trace the nervous fyltem. Analogy will not 

 allow us to doubt its exiftence, when the parts which ac- 

 company it uniformly exift. 



Thus the flukes (fafciola) having veflels, or liver, &c. 

 muft be fuppofed to have nerves alfo, though we have 

 hitherto been unable to demonltrate them. We even doubt 

 not the exiftence of a nervous fyftem in feveral intcftinal 

 worms, particularly thofe which have a cylindrical form, 

 which we fuppofe to have a medulla nearly fimilar to that 

 defcribed in the large afcarides. It is found in the gor- 

 dius ; why fhould it not exift in the cchirrorhynchus, 

 ftrongylus, &c. &c. ? 



But there are animals, in which analogy affords us no 

 afiiftance, to whom we cannot afcribe a nervous fyftem, 

 unlefs we dittinttly obferve it: there are fome inteftinal 

 worms, very different in form from thofe we have mentioned, 

 and the greater part of zoophytes. 



The afterias has parts very fimilar to nerves ; but Gal- 

 vanic experiments ought to be made on living individuals, 

 to prove completely their nature. Round the oefophagus 

 we obferve a girth of a foft whitifh fubftance, which pro- 

 duces ten filaments, two to each of the branches, whieli 

 form the body of the ilar. The two filaments belonging to 

 each branch having arrived at the bafe of the ofTeous and 

 articulated ftalk, which ferves for the principal fnpport of 

 the animal, unite to form a fhort cord, which extends di- 

 rcftly from one to the other : they afterwards both con- 

 tinue along the ftalk to the extremity of the branch, dimi- 

 nilhing always in thicknefs. At the place where they are 

 united, each produces a fafciculus of fdanients, which are 

 diftributed to the ftomach, which, in thefe animals, is 

 fituated in the midft of the body, between the five branches. 

 The appearance of all thefe filaments is rather tendi- 

 nous than nervous, and that circumftance chiefly has hi- 

 therto prevented us from forming a decided opinion of their 

 nature. 



In the Holalhuria, properly fo called, among which we do 

 not include cither the thalia, or the holothuria phyfeter of 

 Linnsus, we find fomething fimilar to what we have de- 

 fcribed in the afterias ; but the appearance of the cord is 

 much more nervous, and this is a flrong confirmation of our 

 conjefturcs. 



The parts we allude to are feen moft diftinftly in the 

 fpecies of holothuria which have five longitudinal pairs of 

 mufcles, as the priapus and pentacla. Between the two muf- 

 cles, which compofe each pair, there is extended a white 

 cord, (lightly ferpentine, and marked by tranfverfe rings, 

 like common nerves. The five cords enlarge as they pro- 

 ceed towards the (fifophagus, where they feem to unite and 

 furround the canal. 



The Sipunculus is more fimilar to the holothuria than to 

 any other animal, though naturalifts have hitherto placed 

 them next the lumbricus. They have only a fingle whitifh 

 cord, but it completely refembles thofe of the holothuria, 

 and it proceeds, in the fame manner, to embrace the oefopha- 

 gus by its anterior extremity. 



If the parts now mentioned arc real nerves, it will be 



necefTary to feparatc the cchino-dermala from the other zoo- 

 phytes, and eflablifli tliem as a diftinft clafs. 



In the Sea Urchins [Echinus), nothing fimilar to nerves 

 has been obferved : tlie fame remark may Ise extended to the 

 aftinix and medufae.* 



With refpeft to the polypes, both the frefh -water kind 

 and thofe which belong to the corals, Sec. we have already 

 obferved that their bodies exhibit only a gelatinous and ho- 

 mogeneous pulp, in which no particular arrangement of 

 organs can be difcerned. All thefe animals have however 

 diftinft fenfations : their fenfe of touch is very delicate j 

 they not only perceive the motions which agitate the water 

 in which they live, but they completely feel the degrees of 

 heat and light. The expanfion of the aftiniae correfponds 

 precifely to the fercnity of the atmofphere. The liydra 

 perceives very diftindly the prefonce of light ; prefers it, 

 and conftantly turns towards it. The microfcopic animals 

 appear to approach in fome meafure the nature of polypi, 

 by their uniform and gelatinous ftrufture. There are fome, 

 however, in which we obferve a more complicated organiza- 

 tion, and feveral kinds of internal vifcera ; but it will be 

 obvious, that we have no means of afcertaining whether they 

 poffefs a nervous fyftem. 



Organs of Senfe 'l"he eye. 



The cephalopodous moUufca have two eyes fituated at 

 the fides of the head, under the tentaculated arms. Moft 

 of the galleropoda have alfo two eyes, but very fmall, and 

 placed either on a level with the head, or on fome of the 

 flclhy and moveable tentacula. In fome they are fituated 

 at the bafe of thefe tentacula; in others at the middle, or 

 the point. In all this order, only the clio, fcyllea, and ler- 

 nea, want eyes. 



No eyes are found in the acephalous moUufca. 



Among the articulated worms there are fometimes found 

 fmall tubercles, whicli have been regarded as fimple eyes, 

 in confcquence of their refemblance to thofe of infefts. 

 Some leeches have two, four, fix, or eight : in fome of the 

 ncreids we find two or four : in fome naiades only two, 5:c. 

 No parts that can be compared to eyes have hitherto been 

 obferved in any zoophyte. 



']'he cephalopodous mollufca, particularly the calniar, 

 have very large eyes ; on the contrary, in fuch of the gaf- 

 teropoda as poffefs eyes, they are fcarcely vifible. 



The eye of the cuttle-fifh has no cornea, nor aqueotis 

 humour : the anterior aperture of the fclerotic is not filled 

 up, and the cryilalline projects acrofs it. Under the con- 

 junitiva, however, 'a particidar membrane is obferved, dry, 

 fine, and tranfparent, enveloping the fclerotica itfelf, and 

 fupplying, by its anterior part, the place of the cornea. 

 Tfiis conjundliva is eafily feparahle from the eye, as in fer- 

 pcnts. The cryftalline is fphcrical, as in animals which fee 

 in water ; and luird in confiftence. The itruiturc of the 

 fclerotic is fingular, being much removed polleriorly from the 

 globe of the eye. The large ganglion of the optic nerve, 

 and feveral other glandular parts, are fituated betwien them. 

 The fclerotica, tlierefore, forms pofleriorly a truncated cone, 

 the pointed part of which is directed to the bottom of the 

 orbit : to this portion the mufcles are attached. "^I'lie an- 

 terior part nearly fliuts the globe of the eye. It is very 

 foft and vifcous ; eafily feparatcd, and prefents a coarfe fclt- 

 likc texture, which becomes firmer in fpirits of wine. In 

 fome fpecies it has a mct.iUic brilliancy. As there is no 

 cornea, the fclerotic is wanting oppofite to the cryilalline ; 

 but the hole is not fufficiently large to admit a view of the 

 iris without diffcftion. 



Tlie internal furface of the choroid is of a purple-red 

 colour, The ufe of the ciliary proccflts, in retaining the 



cryftalline, 



