56 HUMBLE CREATURES. 



the body, is not only contractile, but at the same time 

 divided into a series of chambers (PL III.) opening 

 into one another by means of valves ; the portion of 

 the dorsal vessel, however, which passes through the 

 thorax, is a simple contractile tube resembling that 

 found in the Worm. The blood is forced, by the con- 

 tractility of the dorsal vessel, from the tail towards 

 the head, first passing from chamber to chamber in 

 the abdominal segment ; for the valves which connect 

 these are so constructed as to admit of its flowing 

 forward, but not allowing it to return. From the 

 abdominal chambers it enters that portion of the 

 dorsal vessel situated in the thorax, and is thence pro- 

 jected forward into the head. A ventral tube is also 

 present, which, as in the Worm, conveys the blood 

 from the head backwards in the direction opposed 

 to that which it pursues in the dorsal heart; but, 

 in consequence of the opaque covering of the Fly 

 (and other insects), anatomists have not been able 

 fully to trace the circulation. A surprising circum- 

 stance is, however, known with regard to this func- 

 tion ; and that is, that in some portions of the body 

 the blood circulates in cavities, unconfined by any 

 vessels whatever, and bathes the different viscera. It 

 then collects in the vicinity of the dorsal vessel, the 

 chambers of which are furnished with valves at each 

 side (as well as those which connect one chamber 

 with another) ; and this second set of valves is also 

 so constructed that they admit the blood from with- 

 out, but prevent its egress. It is also ascertained 



