82 INTERNAL ANATOMY OF INSECTS. 



spires through the intestinal canal into the general cavity 

 of the body, where it receives oxygen from the air-vessels, 

 and is ' fitted for nutrition a . In these animals it is ac- 

 companied by a long dorsal vessel, the first step towards 

 a heart, which alternately contracts and dilates with an 

 irregular systole and diastole, but appears to have no 

 vascular system connected with it, though in their pre- 

 paratory states it has an extra-vascular circulation which 

 ceases in the perfect insect. Again : in others, as the 

 Tubicoles, Annelida, &c., a real circulation has been 

 discovered ; that is to say, a system of veins and arte- 

 ries, but unaccompanied by a muscular heart b . In the 

 Arachnida and Branchiopod Crustacea the long dorsal 

 vessel is also found ; but in these it is connected with 

 an arterial and venous system, which receives, distri- 

 butes, and returns the blood c . It has therefore now 

 become a true hearty and there is a regular circulation ; 

 and in the Decapod Crustacea the dorsal vessel is con- 

 tracted into an oval form, and placed nearly in the centre 

 of the trunk d . In the great majority of invertebrate ani- 

 mals the blood is white, but in the Annelida, to which 

 Class 'the common dew- worm belongs, a curious ano- 

 maly takes place for it is red e . Thus a gradual ascent 

 is made to the circulating system of the vertebrate and 

 red-blooded animals. In all, however, the blood is the 

 principal instrument of nutrition and accretion ; and is 

 on that account properly so denominated, though not 

 connected with a circulating system. 



Having given you this general outline of the means 



Herold SchmetterL 25. note *. VOL. III. p. 53. 



b N. Diet. d'Hist. Nat. vii, 313. Cuv. Anat. Comp. iv. 411. 



c Ibid. 419, 407. d MM. * Ibid. 410. 



