444 ARTICULATA. 



which gives off arterial branches, and receives venous ones 

 from them ; but their mode of respiration varies, some of them 

 still having true pulmonary organs which open on the sides 

 of the abdomen, while others receive air by tracheae, like 

 Insects. In both of them, however, we observe lateral open- 

 ings or true stigmata. 



The Insecta constitute the fourth class of the Articulata, 

 and the most numerous of all the animal kingdom. With the 

 exception of some genera, the Myriapoda, in which the body 

 is divided into numerous and nearly equal parts, it is always di- 

 vided into three portions : the head, furnished with the anten- 

 nse, eyes and mouth : the thorax, to which are appended the 

 feet and wings, when they exist ; and the abdomen, which is 

 suspended behind the thorax and contains the principal viscera. 

 Those which have wings, only receive them at a certain age, 

 and frequently pass through two more or le§s different forms 

 before they assume that of the w^inged insect. In all their 

 states they respire by tracheae ; that is, by elastic vessels which 

 receive air through stigmata pierced on their sides, and dis- 

 tribute it by infinite ramifications to every part of the body. 

 A vestige of a heart only is perceptible, consisting of a dorsal 

 vessel which experiences an alternate contraction and dilata- 

 tion, but to which no branch has ever been discovered, so that 

 we are forced to believe that nutrition is effected in this class 

 of animals by imbibition. It js, probably, this sort of nutri- 

 tion which necessitated the kind of respiration proper to in- 

 sects; for as the nutritive fluid is not contained in vessels(l), 

 and could not be directed towards pulmonary organs in search 

 of air, it was requisite that this air should be diffused through- 

 out the body to reach the fluid. This is also the reason why 

 insects have no secretory glands, but are provided with mere 

 spongy vessels, which, by the extent of their surface, appear 



(1) M. Carus has observed regular movements in the fluid M"hich fills the bodies 

 of certain larvje of Insects; but this movement does not take place in a system of 

 closed vessels, as in tlie superior animals. See his ti-eatise entitled " Discovery of 

 a simple circulation of the Mood, &c." in Gennan, Leipzig, 1827, 4to. 



