ORGANS OF CIRCULATION. 67 



dorsal vessel ( l ) by modern writers, I shall use the 

 term heart, which, besides being again introduced by 

 Meckel, Herold, and Straus, is less repulsive to a 

 beginner ; and I shall endeavour to describe is so as 

 to prevent misconception. 



The chief resemblance which the heart in insects 

 bears to that of other animals, is its containing a 

 fluid, and its regular beating, as may readily be 

 observed in smooth caterpillars, in which it is of large 

 size. Lyonnet counted twenty to one hundred beats 

 in the minute in the goat caterpillar, caused, it has 

 been said, by the alternate contraction of a number 

 of muscles ranged along its sides. It differs most 

 essentially from the heart in other animals, in having 

 no visible inlet or outlet in the form of veins or arte- 

 ries. Consequently, there is not, and cannot be, any 

 real or direct circulation of blood in insects, though a 

 claim to the discovery of such a circulation has been 

 lately made, upon very slight and vague grounds, 

 by Professor Carus, of Dresden, and too hastily 

 admitted by Mr. Spence, though the claim is not new, 

 having been made by Compare tti, on the faith of 

 minute dissections. 



The heart lies along the whole extent of the back, 

 from the head to the vent, immediately under the 

 skin and muscles. It is in form of a cylinder con- 

 tracted at the two ends, but usually narrower towards 

 the head, and wider towards the vent. 



It is composed of two membranes ; an outer of cel- 

 lular texture interlaced with numerous airpipes( 2 ), and 

 an inner of muscular texture. 



Within it is filled with a fluid, transparent, eoagu- 

 lable, readily drying, and, when dry, having the look 

 of gum, its colour being sometimes strong and some- 

 times greenish, orange yellow, or dull brown. Without, 



(1) Latin, Vesicula dorsalis. (2) In Latin, Tracheae. 

 E 2 



