THIRD 

 GREAT DIVISION 



OF THE 



ANIMAL KINGDOM. 



(CONTINUED.) 



CLASS III. 



INSECTA. 



Insects, which form the third class of articulated animals provided 

 with articulated legs, have, besides, a dorsal vessel analogous to the 

 vestige of a heart, but totally destitute of any branch for the circula- 

 tion *. They respire by means of two principal tracheae, extending, 



Anatomists are greatly divided with respect to the nature of this organ ; some 

 consider it as a true heart ; others, among whom is the Baron Cuvier, deny it this 

 quality, an opinion which appears to us to be fully confirmed by the admirable re- 

 searches of M. Marcel de Serres " Memoire Sur le Vaisseau Dorsal des Insectes " 

 published in the Me"m. du Mus. d'Hist. Nat. According to the latter it secretes 

 fat, whieh is subsequently elaborated in the adipose tissue which surrounds it. 

 Lyouct says that it contains a gummy substance of an orange colour. Some very 

 recent observations appear to establish the existence of certain very small vessels ; 

 but in addition to the fact that this circulation must be very partial. Insects 

 would still greatly differ, in this respect, from the Crustacea, inasmuch as the 

 blood does not return to the heart. M. Straus in his report Bullet. Univers., de 

 M. le Baron de F^russac on a Memoir of M. Harold on this subject, has inti- 

 mated his own opinion on the matter as deduced from his anatomical investigations 

 of the Melolontha. " The dorsal vessel," says that gentleman, *' is the true 

 heart of Insects, being, as in the higher animals, the locomotive organ of the 

 blood, which, instead of being contained in vessels, is diffused throughout the 

 general cavity of the body. This heart occupies all the length of the back of the 

 abdomen, and terminates anteriorly by a single non-ramified artery which carries 

 the blood into the head where it diffuses it, and whence it returns into the abdomen 

 in consequence of^its accumulation in the head, to again enter the heart ; to this 

 all the circulationTn Insects is reduced, they having merely a single artery without 

 hnmche* and no veins. The ul;r of the heart are not muscular as is asserted by 

 Hit-old they are merely fibrous ligaments which keep the dorsal vessel in its place. 

 The heart, that is to say the abdominal part of the vessel (in the Melolonlha rulyaris) 

 is divided, internally, into eight chambers, separated from each other by two con- 

 verging valvulie, which allow the transmission of the blood from behind forwards, and 

 from one chamber to another, into the artery which runs to the head, but which 

 prevent it from retrograding. At the lateral and anterior part of each chamber, are 



