VERMES. 



to the general covering ; under the nervous collar of the 

 cefophagus it produces a fecond, which goes backwards to 

 the left in this covering ; then immediately afterwards a 

 third, whicli goes to the right for the penis. The trunk is 

 then bifurcated, and loll in and about the mcuth. 



The lung lies on the front of the body in the Aug, and 

 the heart is placed immediately under it. The innumerable 

 ramifications fpread over the internal furface of the lung all 

 end in the auricle, and the latter in the ventricle placed 

 under it, and producing behind two large arteries. One 

 fuddenly turns forwards to the mouth, the generative organs 

 and the general covering ; the other goes direttly back- 

 wards, and is dillributed to all the vifcera. 



The circulation of the pleurobranchus much refcmbles 

 that of the aplyfia. But, as the heart is placed more for- 

 wards, the pollerior artery is the largell of the three, fince 

 it has more parts to nourilh. 



In the teilaceous galleropoda, the heart and its auricle 

 are fituated in the bottom of the great pulmonary cavity, 

 which occupies the upper part of the front of the body, to- 

 wards the edge of the fhell. The lung, whatever may be 

 its form, receives the blood of the body, and a particularly 

 large portion from the laft part of the inteftine, which runs 

 clofe on the furface of the pulmonary cavity, opening fome- 

 times within it, fometimes at its edge. Having pafled 

 through the lung, the blood enters the auricle, goes thence 

 into the ventricle, from which it is fent over the whole body 

 by arteries, which vary according to the general form of the 

 animal. 



The branchia; form a feries all round the body, under the 

 cloak, in the patella. The pulmonary vein is alfo difpofed 

 in a circular manner, coUetls the blood from all the branchial 

 plates by many fmall veins, and carries it to the heart, 

 which is fituated above the head, and diftributcs it over the 

 whole body. 



Acephahui MoUuJca. — In fuch of thefe as liave the heart 

 in the back, and traverfed by the reclum, it is perfedly 

 fymmetrical, oval, broader behind, and accompanied by an 

 auricle on each fide. Their branchia- form tour parallel 

 plates : each auricle receives the blood from tlie two 

 branchia of its own fide, and tranfmits it to the heart. 

 Thefe auricles are triangular, broad towards the branclii<e, 

 and pointed towards tlie heart : fometimes they liave a kind 

 «f criliac, fufceptible of dilatation. Their fides are tranf- 

 parent, and poffels few projetting threads. Their openings 

 into the ventricle are furnifiied with valves, which allow the 

 blood to pafs only from the auricle to the ventricle. The 

 latter is much flrongcr than tlie auricle : its fides arc opaque, 

 and furniflied with numerous flefhy columns. The blood 

 goes from it by two arteries fituated at its two extremities ; 

 thefe follow the reftum, one afcending towards the head, 

 the other defcending to the anus. Such is the heart of the 

 anodontites, or frefh-water mufclc, of the venus, maftra, car- 

 dium, folen, pholas, mya, and apparently of all the equivalve 

 bivalves. 



But the bivalves with unequal (hells, at lead the oyftcrs 

 and the peftens, have the heart differently placed : it occu- 

 pies a cavity between the niafs of the liver and the mufcle 

 that clofes the fliell ; and is directed from behind forwards, 

 or from the back to the branchix, and not, as in other bi- 

 valves, from above downwards, or from the anus to the liead. 

 In this cafe the auricles, or rather the fiugle bilobed auricle 

 is fituated before the heart, and not at the fide. This is 

 jremarkable hi the oyfter on account of its thicknefs, and 

 deep red colour. It receives the blood from the branchiiv, 

 and the heart dillribulcs it to the body by two vcfTels which 

 pafs out at the cstremity cppofilc to the auricle, and go, 



one upwards to the liver, the other downwards to the 

 mufcle. 



Each branchia has an infinite number of fmall, llrajglit, 

 parallel vefTels, terminating perpendicularly in a larger one 

 at the back of the branchia : thefe dorfal trunks convey the 

 blood to the auricle. But each branchia has at tiie fame 

 time another feries of fmall vcffels, fimilar and parallel to the 

 firfl, and pouring the venous blood into their extremities. 

 Tills blood is brought by another veflel at the back-of each 

 branchia, which velFel receives the veins of the body. 



The circulation is carried on in the pteropnda, as in the 

 gafteropoda, by a fimple heart, with one auricle, which re- 

 ceives the blood from the lung, and tranfmits it to the body. 

 Thefe things may be feen in the hyalasa and piicumodermon. 



Cuvier Rates, that he has diffefled only one genus of 

 bracliiopoda, and found two diitinft hearts, both aortic, 

 that IS to fay, receiving blood from the lung, and fending it 

 to the body. 



Tlius we find that the whole clafs of moUufca pofleffes a 

 circulation as complete as any vertebral animal ; and that 

 this circulation is double. When there is only one ventricle, 

 it is aortic, and not pulmonary ; when more than one, they 

 are feparate, and form fo many diftinft hearts. The pafiage 

 from tlie arteries to the veins, in the Lttle as well as in the 

 great circulation, is as evident as in animals of the higher 

 clafies. 



Blood and Circulation of Worms. — The blood is tranfpa- 

 rent, or at molt a little blueifii, in moUufca and cruilacea. 

 The fuppofed red blood of fomc of the firll claf^s is merely 

 a fecretion. But the entire clafs of articulated worms, both 

 marine and terrefl:rial, has the blood more or lefs red, and 

 often of as deep a tint as in any vertebral animal. It may 

 be feen in the genera lumbricus, hirudo, naias, nereis, aphro- 

 dite, amphinomia, amphitrite, terebella, and ferpulu. But 

 the lumbricus marinus (arenicola) exhibits moll plainly, not 

 merely the colour of the nutritive fluid, but alfo its courfe 

 and dircftion : the yellow colour of the inteftine and the grey 

 colour of the parietes of the body allowing all the veffiTs to 

 be perfedlly diftinguiftied. 



A large velTel, diminifhing in fize at the two ends, lies 

 along the back, between the branchix. It fends forward 

 the blood by its anterior origin, and receives fifteen lateral 

 veflels on each fide, one from each branchia. They bring 

 the blood from thofe organs, and are to be regarded as pul- 

 monary veins : when the branchiae contradl, the large veflel 

 is diftended. The blood is carried back to the branchiae 

 by vefl^els fimilar in number to the preceding, but not all 

 arifing from a fingle trunk. The nine firll proceed from a 

 large veflel fituated upon the inteflinal canal immediately 

 under tlie one firR defcribed. The others come from the 

 back part of a vcflll parallel to tlie two firft, but fituated 

 under the inteltinal canal. Thefe two great longitudinal 

 trunks fend all their blood to the branchis ; they reprefent 

 both venK cavx and pulmonary arteries ; for thofe branches 

 which do not go to the lungs are veins returning the blood 

 from the various organs. Thefe branches of the vena cava 

 in the lumbricus marinus are fpread over the yellow furface 

 of the inteftinal canal with an admirable regularity ; and 

 the beauty of the arrangement is heightened by the Iplen- 

 dour of their purple colour. 



All thefe branches arife, in the firft inftance, from two 

 veflels, which proceed along the fides of the inteftinal canal, 

 and perform the office of an aorta. They afcend as far as 

 the lower part of the cefophagus, and then arc bent to 

 communicate with the great pulmonary vein, with which the 

 defcription began. At this communication there in a fwell- 

 ing, which cshibils to the naked eye more marked conlrac- 

 F 2 tions 



