INTERNAL ANATOMY OF INSKCTS. 85 
The fluid which this vessel contains is very abundant ; 
in the animal it appears colourless and transparent like 
water, but when collected in drops it becomes more or 
less yellow, and even orange a . Examined under the 
microscope it appears filled with a prodigious number of 
transparent globules, of incredible minuteness b . When 
mixed with water, which it does readily, its globules lose 
all their transparency, and coagulate into small clammy 
masses. After evaporation it becomes hard, and cracks 
like gum, as blood does also. This gummy substance is 
so abundant, that the fluid contained in the dorsal vessel 
of the caterpillar of the Cossus yields a mass of it of the 
size of a grey pea c . 
From the situation of this dorsal vessel, which is pre- 
cisely the same with that of the heart in Arachnida and 
the Branch iopod Crustacea ', and from the systole and 
diastole which keep its fluid contents in constant motion, 
who can wonder that the physiologists who first disco- 
vered it, reasoning analogically, maintained that it was a 
true heart ? But modern comparative anatomists, and 
those of the highest name, from the absence of a vascu- 
lar system for a circulation, have contended that it is 
not a true heart, but an organ appropriated to other 
purposes : a third hypothesis, and intermediate between 
these two, has very recently been promulgated, that 
the organ in question, namely, is a real heart, and in the 
preparatory states of insects, the centre of a real circu- 
lation, which, in the imago state, ceases with the full de- 
velopment of the wings ; but that this circulation is ex- 
a Lyonet Ibid. 426. Cuv. Anal. Comp. iv. 419. 
b Lyonet says (426), " au-dela. de tiois millions dc fois plus petits. 
qu'un grain de sable" !! ° Ibid. 
