174 THE VITAL FUNCTIONS. 



ceivable purpose is that of determining the motion of the 

 contained fluid in one constant course; a purpose necessarily- 

 incompatible with its supposed alternate undulation in op- 

 posite directions, from one end of the tube to the other. 

 These valves are exhibited in Fig. 336, in a still more mag- 

 nified view of a longitudinal section of the dorsal vessel, 

 showing tlie semicircular folds (s, s) of its inner membrane, 

 which perform the function of valves by closing the passage 

 against any retrograde motion of the fluid. This discovery 

 of valves in the dorsal vessel, again made the balance of pro- 

 bability incline towards the opinion that it is the agent of 

 some kind of circulation. 



All doubt as to the reality of a circulation in insects is 

 now dispelled by the brilliant discoveries of Professor Ca- 

 rus, who, in the year 1824, first observed this phenomenon 

 in the larva of the Agrion puella. In the transparent parts 

 of this insect, as w^ell as of many others, numerous streams 

 of fluid, rendered manifest by the motions of the globules 

 they contain, are seen meandering in the spaces which in- 

 tervene between the layers of the integument, but without 

 appearing to be confined within any regular vessels. The 

 streams on the sides of the body all pass in a direction back- 

 wards from the head, till they reach the neighbourhood of 

 the posterior end of the dorsal vessel, towards which they 

 all converge; they are then seen to enter that vessel, and to 

 be propelled by its pulsations towards its anterior extremi- 

 ty, where they again issue from it, and are subsequently di- 

 vided into the scattered streams, which descend along the 

 sides of the body, and which, after having thus completed 

 their circuit, return into the pulsating dorsal vessel. 



This mixed kind of circulation, partly difl'used and partly 

 vascular, is beautifully seen in the larva of the Ephemera 

 marginata,'^' where, besides the main current, which, after 



• This insect is figured and described in Dr. Goring- and Mr. Pritchard's 

 *' Microscopic Illustrations," and its circulation is very fully detailed, and il- 

 lustrated by an engraving- on a large scale, by Mr. Bowerbank, in the Ento- 

 mological Magazine, i. 239; plate ii. 



