USE OF THE MICROSCOPE I XT ZOOLOGY. 59 



is a momentary check, and the blood-corpuscles col- 

 lect together in one spot. Perhaps the frog is too 

 tightly fastened, or alarm may have interfered with 

 the heart's action. You will notice several dark 

 opaque bodies in the substance of the frog's feet ; 

 these are pigment cells. 



The water larvae of various kinds of insects, small 

 Crustacea, such as Daphnia pulex, &c., will reward you 

 for a patient study of the circulatory system in these 

 creatures. Both in the larva, pupa, and imago stages 

 insects have not a heart, but a long dorsal vessel, which 

 is really made up of a series of contractile cavities, 

 one for each segment of the body, opening one into 

 another from behind forwards, the whole being divided 

 by valvular partitions. This is the typical form of the 

 circulatory system. It must be confessed, however, 

 that there is much difficulty in always making out 

 these valvular partitions. The larvae of any of the 

 EphtmcricUz are capital objects for examination. A 

 smaller specimen laid upon a glass slide, with a drop 

 of water and a thin glass cover over it, will serve well 

 to show you the circulation of blood in insects. You 

 will notice that the blood is almost colourless, that the 

 corpuscles are oat-shaped. " The current enters the 

 dorsal vessel at its posterior extremity, and is propelled 

 forwards by the contractions of the successive cham- 

 bers, being prevented from moving in the opposite 

 direction by the valves between the chambers, which 

 only open forwards. Arrived at the anterior extremity 

 of the dorsal vessel, the blood is distributed in three 

 principal channels ; a central one, namely, passing to 

 the head, and a lateral one to either side, descending 

 so as to approach the lower surface of the body. It 

 is from the two lateral currents that the secondary 

 streams diverge, which pass into the legs and wings, 

 and then return back to the main stream. It is from 



