INSECTS. 



ncfted together, or fixed in any way except'by the trachea. 

 This appears clearly to be a neceffiiry confequence, and 

 therefore a very probable indication of the ablence of blood- 

 veffels. When the powerful afrents of circulation exift, they 

 convey with facihty the nutritive fluid to the moft remote 

 points of the glands ; the intertexture of the blood-veflels 

 forms a thick and clofe tifiue, in which the peculiar veflels of 

 the gland are pliced. When, on the contrary, there is nei- 

 ther heart ni-r blood-veflcls, nor any force to impel the fluid 

 towards the decretory organs, the latter require a more 

 powerful attradive force ; and as this can be exerted only 

 by means of the tiflue compoiing their fides, it becomes ne- 

 cen"ary that they fliould be unattached,loofe,long, and flender, 

 in order to augment their furface. 



Organs of Clrculalion in tin Crujlacea The heart of the 



crultacea decapoda is very diflferently formed from that of 

 the branchiopoda. In the former it is oval, circumfcribed, 

 and placed nearly in the middle of the thorax ; in the others 

 it is elongated, and extends from one end of the body to the 

 other, fo as to form apparently a connecting link between the 

 heart of the decapoda and the dorfal veffel of the other in- 

 fects. In this pouit of view it has led fome naturalifl:s into 

 error: we fliall, however, find a ilniaure which is more 

 analogous to it in the red-blooded worms. 



The decapoda (the crabs, lobfter, cray-fifli, hermit crab, 

 &c.) have an aortic heart, like that of the mollufca. It re- 

 ceives the blood from the branchiae by a large veCel, which 

 lieslongitudinally in the chell to receive this 'blood by lateral 

 veflels. Cuvier itates that this is the cafe in the hermit crab ; 

 but he thinks that in the lob Her the branchial veins form two 

 trunks, which go diredly to the two fides of the heart. 

 When one of the veins of the brarchis is injected, the fluid 

 arrives at the heart by the route jufl; indicated. From the 

 fame poilerior part of the heart arifes an arterial veflel, which 

 gjoes directly backwards, and fupplies the organs of genera- 

 tion and the mufcles of the tail. The anterior portion 

 produces other arteries, varying in number according to the 

 fpecies. 



Each pedicle of the branchia: contains two principal vef- 

 fels ; an artery and a vein. The venous trunks all go to the 

 heart ; ard, as we have already fl:ated, by a fingle trunk in 

 the decapoda : but. in the branchiopoda, where the heart is 

 elongated, they all terminate in it diredly, fo that a pair of 

 veins enters at each ring of the body. The branchial arte- 

 ries do not come from the heart. When the latter organ 

 is injected, the fluid does not enter the branchitc, although 

 it will pafs ealily from the latter to the heart. In the fquil- 

 la fafciata, Fabr., Cuvier obferved a large longitudinal vena 

 cava, extending from one end of the body to the other, un- 

 der the inteftine, and confcquently on the oppofite furface to 

 that occupied by the heart. Its tifl"ue is much thinner than 

 that of the heart, and tranfparent, and it produces as many 

 pairs of branchial veflels as the heart receives from thofe or- 

 gans. This anatomift lias very little doubt that the fame 

 veikl will be found in the decapoda, but he had not examined 

 them for this purpofe fince he firfl. faw it in the other di- 

 vifion. 



The circulation of the criiftacea is then the fame with that 

 of the g ifteropodous mollufca ; vi%. a double circulation, 

 of which the aortic fyllem only poffelfes a ventricle : and 

 even this ventricle does not dcferve the name in the branchio- 

 poda, it fo much i-efemblcs in them a veffel by its elongated 

 form. Under this point of view the circulating fyllem of 

 thefe animals refembks that of red-blooded worms. 



The heart of the cruilacea, even in the decapoda, has no 

 auricle ; and valves have not hitherto been obferved in it 



It IS hardly neceflary to ftate, that the blood thrown by 



the heart into the arteries, pafTes through the veins into i:;:; 

 vena cava. 



The motions of the heart may be feen in thefmall mono= 

 cull of this country, but their extreme fmallnefs prevents us 

 from following the diflribution of the veflels. The anatomy 

 of t;ie molucca crab is not yet known. 



ChculaUon of hifccls. — All infects have in their back a lon- 

 gitudinal veflel, filled with a tranfparent fluid, and for a hir.g 

 time regarded as the heart, after the Itatement of Malpighi, 

 who defcribed it in the filk-worm, and reprefents it as a 

 knotted canal, that is divided at intervals by conilriilions. 

 He conceived that each dilatation was a particular heart, and 

 that thefe diffVrent organs tranfmitted the blood to each 

 other : but he remarked at the fame time that the fucceffion 

 of the pulfations was not regular, and tliat the fluid fon e- 

 times took a retrograde courle. I.yonet has given a better 

 account of this dorfal veflel. It is an uniform canal, pro- 

 ceeding from the head to the oppofite extremity, and enlarg- 

 ing a little in its courfc, but clofed at the two ends. It pof- 

 felles on each fide a certain number of tranfverfe mufcular 

 fafciculi, fomewhat in the form of wings, which are fixed 

 by their oppofite ends to the general covering. Its dilata- 

 tions and contraftions are produced by thefe, which are ex- 

 terior mufcles, and not by its own tifliie. 



Lyonet afi'ures us, that he has not been able to difcover 

 any veflel produced from this and proceeding into the body, 

 although he has defcribed trachex and nerves a thoufand 

 times fmaller than thefe veflels mull be if they exi'l. Cuvier 

 has tried all known methods of injedion, without any 

 greater fuccefs. Swammcrdam indeed mentions that he in- 

 jcded fmall veflels from it in the grylli ; but we cannot help 

 entertaining doubts on this fubjett, until the experiment fliall 

 have been repeated with fuccefs. 



The dorfal veflel of infefts cannot then in any manner per- 

 form the funftions, nor dcferve the name of a heart. Perhaps, 

 like all other parts of this kind in infedls, it is a fecretory 

 veflel : but we are at prefent quite unable to determine what 

 liquor it fecretes, and for what ufe. Its contents are tranf- 

 parent, light yellow, vifcous, mifcible with water, ealily 

 dried, and then becoming hard and cracking hke gum. 



Befides the analogy of its contractions, its fituation might 

 favour the idea of its being a heart. The latter organ is 

 placed near the back in almoil all the mollufca, and in all the 

 cruftacea ; and this is the pofition of an organ, which can 

 hardly be regarded in any other light than as a heart, in the 

 araclinidous infeCTis (aranex, phalangia, and fcorpions). It 

 may be obferved very eafily in the fpiders : and may be 

 feen beating through the integuments of the abdomen in the 

 fpecies which have fmooth bodies. On removing the inte- 

 guments v>'e expofe a hollow oblong organ, pointed at its 

 two ends, and advancing as far as the thorax by its front end : 

 two or three pairs of veflels very manifeilly pafs off from its 

 fides. If we add to this account, the faCl, that the fpiders 

 have no trachcx, but that their refpiralion is circumfcribed 

 within a fmall number of veficles, and that they appear to 

 have glands, we fliall be induced to believe that their circu- 

 lation is more complete, and more analogous to our's than 

 that of other inleds. 



Rifflratory Organs. — The clafs of infefts exhibits two 

 very different arrangements of the organs belonging to this 

 function : the cruftacea have gills, refembling thofe of fiflies 

 and of many mollufca ; other mlVds have air-veflfels dilhi- 

 buted to all parts of their bodies. The brantluK of the 

 former extract air from the water in which they live, in the 

 fame way as the gills of fiflies aC\. The rcfpiration of infeds 

 produces in the air the fame efleds as that of the warm- 

 blooded animals. Experiments fliew us that oxygen is con- 

 fumed 



