220 



THE VITAL FUNCTIOMS. 



curve (a) which it describes as it bends downwards 

 in its course towards the head. 



The valves in the interior of the vessel are shown 

 in Fig. 336, w liich is a still more magnified view of 

 a longitudinal section of the dorsal vessel, showing 

 the 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. 



Professor Cams first observed the circulation in 

 the larva of the Agrion puella. In the transparent 

 parts of this insect, as well 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 intervene 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 direc- 

 tion backwards 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 extremity, 

 where they again issue from it, and are subse- 

 quently divided 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 diff'used 

 and partly vascular, is beautifully seen in the 

 larva of the Ephemera marginata* where, besides 



* This insect is figured and described in Dr. Gorino- and Mr. 

 Pritcluird's " Microscopic Illustrations," and its circulation is very 

 fully detailed, and illustrated by an engraving on a large scale, by 

 Mr. Bowerbank, in the Entomological Magazine, i, 239 ; plate ii. 



