364 MANIPULATION. 



according to Mr. Pritchard, it may be witnessed in the Perla 

 viridis and Semblis bilineata, when they have just emerged 

 from the chrysalis. The most favourable subjects for its exhi- 

 bition are those found in water, viz., certain larvae, together 

 with small Crustacea and annehdes ; these may be placed for 

 examination either in the animalcule cages described at 

 page 130, or in the water trough shown at fig. 93, or even 

 in any suitable tubular or drilled cell, and covered over 

 with thin glass. Amongst the most beautiful are the larvse 

 of the following insects, viz., the Ephemera or day-fly, the 

 Plumed gnat, the Hydrophilus caraboides, and a Dragon fly 

 named Agrion puella; and amongst the Crustacea may be 

 mentioned the ordinary Water-flea, or Monoculus, the Daphnia 

 pulex, or Arborescent water-flea, both of which are very com- 

 mon in stagnant pools, together with the fresh-water shrimp, 

 and various species of Oniscus or Water-hog, aU of which may 

 be examined in the water trough or in a large animalcule cage. 

 The circulation in the larva of the Ephemera marginata has 

 been accurately described in the first volume of the Entomo- 

 logical Magazine, by Mr. Bowerbank; where also may be 

 seen a well-executed figure of the larva, as shown by the 

 microscope. The blood is colourless, and consists of numerous 

 oat-shaped cells or particles not contained in vessels, but which 

 are sent to all parts of the body by the pulsation of a large 

 dorsal vessel or heart, extending nearly the whole length of 

 the trunk, and furnished with valves of a peculiar construc- 

 tion, about equal in number to the segments of the body. 

 Besides the circulation of the blood in this animal, there are 

 many other points of interest which may be seen with the 

 half-inch object-glass. The structure of the valves can only 

 be well seen when they move slowly, and then only in the 

 three or four last segments of the body, when the vital powers 

 are nearly exhausted. In the Daphnia pulex, the oval dorsal 

 vessel or heart may be seen pulsating rapidly on its least 

 convex side or back, and the corpuscles of blood may be 

 noticed in its immediate neighbourhood in an active state of 

 movement by a magnifying power of two hundred, or the 

 quarter-of-an-inch object-glass, when the animal is confined 



