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 heart, and there is a regular circidation ; 
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 raZ 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 
» Hcrold Schmetterl. 25. note *. Vol. III. p. 53. 
b N. Did. (THist. Nat. vii. 313. Cuv. Anat. Comp. iv. 411. 
1 Ibid. 419. 407- " Ibid. e Ibid. 410. 
