INTERNAL ANATOMY OF INSECTS. 81 
tinal canal into the general cavity of the body, where it 
receives oxygen from the air-vessels, and is fitted for 
nutrition*. In these animals it is accompanied by a long 
dorsal vessel, the first step towards a heart, which alter- 
nately contracts and dilates with an irregular systole and 
diastole, but appears to have no vascular system con- 
nected with it. Again: in others, as the Tubicoles, An- 
nelida, &c., a real circulation has been discovered; that 
is to say, a system of veins and arteries, but unaccom- 
panied by a muscular heart®. 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, distributes, and returns 
the blood*. It has therefore now become a true heart, 
and there is a regular circulation ; and in the Decapod 
Crustacea the dorsal vessel is contracted into an oval 
form, and placed nearly in the centre of the trunk?. In 
the great majority of invertebrate animals the blood is 
white, but in the Annelida, to which Ciass the common 
dew-worm belongs, a curious anomaly takes place—for 
it is red®. ‘Thus a gradual ascent is made to the circu- 
lating 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 by 
which the blood is distributed in the different Classes of 
animals, I shall now confine myself to the case of insects 
4 Herold Schmetterl. 25, note *. Vot. III. p. 53. 
> N. Dict. d’ Hist. Nat. vii. 313. Cuv. Anat. Comp. iv. 411. 
© Ibid. 419, 407. a Thid. © Tbid. 410. 
VOL. IV. G 
