VERMES. 



of the body, in wliich there is always much fluid to be ab« 

 forbed, with the trunks of the great veins. 



Thefe communications are particularly obvious in the 

 cephalopoda, where the principal branches of the vena cava 

 are furniihed with a multitude of bodies refembling ramified 

 glands, and floating loofely in the abdomen. They have 

 tubes manifeftly ending in the trunk of the vein. Fluids in- 

 jeded into the vein penetrate like a dew the extremities o#" 

 thefe ramifications, and pafs into the abdominal cavity. 

 Sometimes air will pafs in the fame way. There mult 

 equally be a paifage in the oppofite direftion. 



Among the gafteropoda, the aplyfia exhibits a communi- 

 cation no lefs free between its veins and the great cavities of 

 the body. If we impel air from the lung into the vense 

 cavx, which are continuous in thefe animals with the pul- 

 monary artery, the abdomen will be diftended. The ori- 

 fices, through which the air efcapes, are vifible to the naked 

 eye : they mull admit liquids from the abdomen, as they 

 allow air to pafs from the veflels into that cavity. 



The paffage of the reftum through the heart in the ace- 

 phala is another point deferving attention. We cannot fee 

 what end this arrangement can ferve, if the nutritive fluid 

 does not find its way through the inteftine into the heart, 

 where it will be mingled with the blood, and fet in motion. 



This manner of viewing the fubjeft coincides extremiely 

 well with the gradation of the organic fyftem3,tin the dif- 

 ferent clafles of animals. Infefts mod probably have no 

 velfel at all (fee In.sects, in Anatomy): it is, therefore, 

 natural to find before them, in the fcale, animals which 

 have veflels of one kind only, and which, therefore, may be 

 arranged between the vertebral divifion poflefllng the two 

 kinds, "u/z. lymphatic and fanguineous, and the infeAs 

 which have none ; unlefs at leall we regard the fecretory 

 tubes as a third order, the mod eflential, becaufe common 

 to all. The moUufca, vermes, and crullacea, feem deftined 

 to hold this intermediate rank. The echino-dermata, and 

 particularly the holothuriae, are of a doubtful kind : their 

 place cannot be yet affigned. 



In the zoophytes, properly fo called, the fubftance of 

 their body forming the fides of the alimentary cavity is im- 

 mediately impregnated with the nutritive fluid. The medufre 

 do not differ in this refpeft from the fimplefl polypes, except 

 that their cavity has numerous tubular prolongations. If 

 thefe inteftinal tubes are to be confidered as veflels, the 

 fl.omach will perform, with refpeft to them, the funftions 

 of a heart. 



Organs of Circulation and Refpiration. — As both thefe kinds 

 of organs exift together in all the vertebral clafles, there can 

 be no variety in their combinations ; but one or the other may 

 be wanting in invertebral animals, fo that we may ellabhfli 

 between them in this refpeft relations, which are very con- 

 ftant in the claflfes, in which thefe organs are perfeftly un- 

 dcrftood. Thus, in the moUufca, the worms with red 

 blood, and the cruftacea, which have a complete circulation, 

 we find circumfcribed branchiae. Infefts have the body 

 nouriflicd by a fluid, which ftagnates inilead of circulating ; 

 and in them refpiration is eff'efted by means of tracheae, 

 which are diftributcd over the whole body. True zoo- 

 phytes, medufse, and polypes, in which the body itfelf 

 forms the fides of the intellmal canal, and direftly abforbs 

 its nourifliment, have no particular organ for refpir.ition. 

 Probably the whole body refpires. 



The moUufca have a double circulation ; that is, all their 

 blood, after circulating through the body, partes through 

 the lungs before it is fit to be circulated again. 



The cephalopoda Iiavc three hearts, two compofcd of a 

 ventricle and an auricle, and one of a ventricle only : the 

 Vol. XXXVII. 



gafteropoda have one, confifting of a ventricle and an au- 

 ricle ; the acephala one, of a ventricle with two auricles ; and 

 the brachiopoda two, of a ventricle without an auricle. 

 This clafs alone, in faft, exhibits nearly as many modifica- 

 tions of the circulating organs, as all the four clafles of ver- 

 tebral animals : thefe modifications, however, have reference 

 to the number and pofition of the auricles and ventricles, 

 not to the courfe of the circulation, which is always double. 

 The cephalopodous mollufca have the moit complicated 

 fyftem of circulating organs of all animals, poffeffing three 

 diftinft hearts, two pulmonary and one aortic. 



The defcending vena cava, formed by the union of branches 

 which return the blood from the head and arms, pafles from 

 the neck, along the front of the liver, tovvards the bottom 

 of the abdominal fac : it receives the hepatic vein in its 

 courfe, and immediately afterwards, that is about the middle 

 of the abdomen, it is bifurcated, each branch going tranf- 

 verfely to one of the lateral hearts ; but before they arrive, 

 they receive various branches from other parts. Thus, di- 

 reftly after their origin from the common trunk, each re- 

 ceives a vein from the inteftines and back of the body ; and 

 at the very point of entering the hearts, each receives another 

 from the lower parts. All thefe veins are extremely thin 

 and tranfparent : tliey are much more capacious and exten- 

 file than the arteries ; no valve can be feen in them, except 

 at the entrance of the hepatic vein. 



The two great tranfverfe branches, which end in tlie late- 

 ral hearts, and all the veins immediately fending in them, are 

 perforated by openings leading into very Angular appendices 

 of a ramified or glandular appearance, fuch as are found in 

 the nervous fyftem of no other animal. 



They are numerous, large, and of an opaque yellowifli- 

 white : only two offices can be afcribed to them ; either that 

 of fecreting fome fluid from the arterial blood, or of ab- 

 forbing the Hquids of the abdomen and conveying them into 

 the veins. The fmall number of their arterial ramifications 

 favours the latter idea : it is fufficient for their nutrition, 

 but not for a fecretion proportioned to their volume. 



The two lateral hearts are placed at the root of the 

 branchia; ; they are more or lefs rounded, with thick, muf- 

 cular, though rather foft parietes, and large flefliy columns, 

 mtercepting numerous fpaces of diff^erent fize. In the icpia 

 oftopus they are of a very deep brown red, as in a red- 

 blooded animal, while all tlie other vifcera, the mufcles, and 

 the aortic heart, are whitilh. 



The entrance of the vein into each lateral heart is fur- 

 niflied with two membranous reftangular valves, fixed at 

 their bales and extremities, and loofe only at the inner edge ; 

 they allow the blood to pafs in, but prevent its return. The 

 pulmonary artery goes out at the extremity of the heart op- 

 pofite to the entrance of the vein. There is no valve at its 

 origin in the oftopus, but in the cuttle -fifli and calmar 

 there are four, fliaped like fmall flefliy tubercles, furround- 

 ing the orifice of the artery, and preventing the return of the 

 blood. They are a little beyond the origin, and in the very 

 trunk of the artery. Tlie latter runs along the external and 

 pofterioredge of the gill, producing as many lateral branches, 

 perpendicular to its trunk, as there are plates of llie gill. 

 Their ramifications and dillribution will be dcfcribed in the 

 article on refpiration. A branchial vein is found on the 

 oppofite or internal and anterior edge of the gill, from which 

 it coUcfts the blood, lleacliiug the lower end of the gill, 

 the vein quits it, and runs tranfverfcly towards tht; middle 

 of the body, a little below and behind the part where the 

 vena cava bifurcates. Here it ends in the third, aortic, or 

 intermediate heart. This heart receives then two pulmo- 

 nary veins, one from each gill, which end each on its own 

 F fide, 



