INTERNAL ANATOMY OF INSECTS. 85 



Thejluid 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- 



3 Lyonet Ibid. 426. Cuv. Anat. Comp. iv. 419. 

 b Lyonet says (426), " au-dela cle trois millions de fois plus petits 

 qu'urv grain de sable " ! ! Ibid. 



