VERMES. 



ber of moUufca ; the fecond is the mod common, and is found 

 in moil mollufca, in worms, and ciuftacca. The mode of 

 refpiration is not well known in the echino-dermata, fo that 

 their organs cannot be claffed with certainty. 



The efFedt of refpiration cannot be eilimated by the co- 

 lour of the blood, except in red-blooded worms, where it 

 is very obvious : it may be feen without hgature or incifion 

 in the branohia; of the lumbricus marinus. But the efFeft of 

 this function on tlie refpired air may be eafily judged : the 

 refearches of Vauquelin and other naturalifts have (hewn that 

 invertebral animals confume oxygen like others, and infeft 

 the refidue with carbonic acid. See Respiration. 



Refp'tratory Organs of the Mollufca. — We meet in this clafs 

 with lungs, with uncovered branchiae, and with branchix 

 contained in a cavity. In the cephalopoda and acephala 

 they are always of the latter kind : the gafteropoda have all 

 three forts. A lung is found in the terreftrial gafteropoda, 

 and in thofe aquatic kinds which are obhged to come to the 

 furface of the water in order to take in air. The principal 

 genera that have it are the f nail (helix), flug(limax), the teila- 

 cella and parmacella, among the terreftrial ; in the onchidium, 

 buHmus of pools (helix ftagnalis), and planorbis, among the 

 aquatic. This lung is a larger or fmaller cavity, commu- 

 nicating externally by a narrow aperture, which can be 

 opened or clofed voluntarily, while the cavity, contradling 

 or dilating at the fame time, expels or admits air. As the 

 parietes are mufcular, and there is no bony ftrufture, there 

 is no other mechanifm than mufcul.ir contraftion. The pa- 

 rietes of the cavity are furniftied with an almoft infinite net- 

 work of blood-venels, ramified in a rather fpongy fubftance. 

 The cavity itfelf is placed on the neck, and opens at the 

 right fide of the cheft, in the fnail, Aug, bulimus, and 

 planorbis ; on the back, and opens on the right fide of the 

 body, in the parmacella ; on the back, and opens back- 

 wards, in the teftacclla ; on the pofterior part of the body, 

 and opens behind, under the edge of the cloak, in the on- 

 chidium. 



The branchia; projefting externally, fometimes reprefent 

 tufts or trees, as in the tritonise, where they form a kind of 

 hedge all round the body, and in the doris, where they have 

 a circular arrangement round the anus, at the pofterior part 

 of the back ; fometimes in fmall laminae or fcales, as in the 

 eolides, where they are difpofed like tiles ,on the back, in 

 the phyllidiae, the ofcabrio, the patcllx, where they form a 

 cordon all round the body, under the edge of the cloak. 

 In the fcyllea they arc pencils of filaments, difperfed over 

 flefiiy plates, or a kind of wings placed on the back. In 

 the glaucus they refemble fins, radiated like a fan : in the 

 pleurobranchus they arc fmall plates, arranged in tranlverfe 

 rows on the two furfaces of a prominent plate at the right 

 fide of the body. 



Teftaceous gafteropoda have prominent branchiae, but 

 fituated in a cavity concealed under the edge of the (hell. 

 The opening is generally very free, and occupies all the 

 upper part of the animal's neck. Often alfo a part of the 

 flefhy edge of the cloak is prolonged into a fmall canal, 

 lodged in a correfponding canal of the (hell, and calculated 

 toconduA the furrounding element into the branchial ca- 

 vity, even while the animal is entirely inclofed in its calca- 

 reous habitation. Thcfe canals are found in all the genera 

 made out of thofe united together by Linnxus under the 

 names bucciuum, murcx, and ftrombus. In moll of the 

 genera the branchix form one or two long ferics of tranfverfe 

 plates, occupying the whole length of tlie cavity, but a part 

 only of its breadth, and rcprclenting, fometimes a priim, 

 fometimes a kind of pen fixed by the whole length of its 

 item. There is a fingle ferics in the murcx tritonis ; a large 



and a fmall one in the buccinum uudatura ; two large ones 

 in the halyotis. 



Some genera however deviate from this general rule : the 

 patella Hungarica, which fcems fo much like the other pa- 

 tellx, has its branchix arranged in fmall long plates, placed 

 within a cavity above the neck, but forming a tranfverfe 

 feries round the edge of the cavity. 



The courfe of the blood, however, is the fame, whatever 

 form the branchix may pofTefs in the gafteropoda : each di- 

 vifion and fubdivifion receives a pulmonary arterial branch 

 from the vena cava, and fends a venous branch into the pul- 

 monary vein, which terminates in the heart. The pofi- 

 tion of the branchix regulates that of the heart, as well as 

 the courfe of the large veffels. 



The branchix of the acephala are formed into plates, each 

 compofed of a double leaf : they iiave a double feries of 

 velfels, very regularly and clofely arranged, like the .teeth 

 of a fine comb, the ftrix being at right angles to the length 

 of the plate. An arter)- and a vein run along the bafis of 

 the plate. The teftaceous acephala have four of thefe 

 plates, inclofed between the two lobes of the cloak, and 

 allowing the foot to pafs between them when there is one. 

 The internal furface of the four triangular plates furround- 

 ing the mouth, and occupying the place of lips or tentacula, 

 is alfo ftriated with veffels fimilar to thofe of the branchix, 

 and may probably aflift in refpiration. 



Poli fpeaks of fmall air-veflels, commencing in the fmall 

 tentacula, ufually fituated at the pofterior edge of the 

 cloak, or round the orifice of the branchial tube : he fup- 

 pofes that they penetrate to a certain refervoir, whence the 

 air pafles into the interior of the branchix. Cuvier has not 

 found this ftrufture, and thinks that refpiration is carried 

 on in the acephala, as in other mollufca and filhes, by the 

 fimple afflux of water over the external furface of the 

 branchix. 



Some genera bring this water to the branchia: by 

 fimply opening the (hell and the anterior edges of the 

 cloak. It is expelled by again ftiutting the ftiell. In 

 the mufcle, which has the wideft opening of the (hell be- 

 hind, the water paffes in and out at this part. When the 

 animal is placed in water, a (light motion of the fluid is per- 

 ceived in this fituation, produced by the procefs of refpira- 

 tion. In the genera which have the cloak prolonged be- 

 hind into one or two tubes, the water enters, and is dif- 

 charged by the tube fartheft from the back, or by the ana- 

 logous canal, when there is only one tube : for it is then 

 divided into two canals. The cardium, venus, maftra, 

 teUina, &c. &c. have two tubes ; the pholas, folen, teredo, 

 mya, S:c. have only one. They can partly withdraw the 

 tubes into the (hell by means of two flat, fan-fhaped, re- 

 traftor mufcles, attached to the lobes of the cloak : but 

 they do not extend them fimply by mufcular aftion ; for 

 they may be feen to increafe in length and breadth both at 

 the fame time in the pholades. 



In the afcidix, which are naked acephala, the branchix 

 do not form four plates, but a fingle large fac, with an ex- 

 tremely fine vafcu'.ar net-work. This bag is filled with 

 water as often as the animal dilates it : the mouth is at its 

 bottom. In the biphori, or falpx, and the thalia, they 

 form only a narrow ribbon, obliquely traverfing the interior 

 of the body : tlie water, in palling through this from before 

 backwards, neccn"arily goes over this ribbon. 



The cephalopoda alfo have their branchix inclofed in a 

 cavity, that is, in the bag forming their body. They are fe- 

 parated from the other vifcera by the peritoneum, and their 

 cavity communicates externally by the funnel under the 

 neck. The water is admitted and expelled by the dilatation 



and 



