THE COASTS OF SICILY. 271 



resemblance to an elastic webbing. These tracheae 

 ramify over the whole body, and, consequently, as 

 Cuvier remarked, the air seems in insects to go in 

 search of the blood, whilst the contrary is the case 

 in other animals. The explanation was logical, for 

 every movement of this liquid seemed as if it were 

 useless, since it could be incessantly revivified on the 

 spot. A more attentive observation has, however, 

 shown that insects possess a true circulation, for 

 a long contractile dorsal vessel here plays the part 

 of a heart. The blood moves freely through the 

 interstices of the organs, yet each one of its 

 portions circulates successively through every part 

 of the organism ; but then the circulation is per- 

 formed almost entirely in lacuncB, We may readily 

 observe, under the microscope, the existence and 

 direction of all these currents, which are made 

 apparent by the globules in the liquid. The cir- 

 culatory circle is therefore incomplete in all the In- 

 vertebrata of which we have spoken, and hence it is 

 the more remarkable that the class of the Annelids 

 should possess an uninterrupted circulation. We 

 undoubtedly meet among the lower forms of this 

 group of animals with many totally deficient in an 

 apparatus of circulation, while some species exhibit 

 a mere rude outline of such a system ; but the greater 

 number certainly possess a perfectly closed system 

 of sansfuiniferous vessels. Even in the Nemertes, 

 whose animal machine presents a most remarkable 

 degree of simplification, the blood pursues its course 

 without ever leaving the contractile tubes within 

 which it is contained. In these animals, however. 



