158 J. 0. WAKELIN BAERATT AND C4E0RGE ARNOLD. 



It will be noticed that no dissolved oi* loosely combined 

 oxygen was obtained from the blood-plasma of Dytiscus 

 and Hydvophil n s. AVhen oxygen was absorbed in vitro 

 the blood-plasma became darkly coloured. It follows, there- 

 fore, that as long as the blood-plasma remains straw-yellow 

 coloured the absence of dissolved oxygen may be inferred. 

 No data ai-e, however, available to indicate the means 

 by which darkening of the circulating fluid is avoided 

 in the living insect. The blood appears to serve solely as a 

 nutritive medium. , The tissue-cells, it may be observed, are 

 in direct relationship to the finest ramifications of the 

 tracheal vessels (9), which penetrate to all parts of tlie body 

 of these insects. From the tracheal vessels the tissue-cells 

 appear to derive their supply of oxygen directly, not being 

 dependent on the mediation of the blood-plasma as in 

 mammals and in animals living exclusively in water. 



The Chakactees of the Blood-cells. 



The blood-cells were studied in films fixed in Fleniming's 

 strong solution, without previous drying, and also in dry films. 

 In addition, Flemming's solution was added to the blood, and 

 the formed elements, after centrifugalisation, embedded and 

 cut in paraffin. 



The stains chiefly used were Heidenhain's iron-alum hasma- 

 toxylin, Breinl's methylenblue-saffranin-orange G. triple stain 

 and basic fuchsin-methylenblue-orange G. triple stain. Intra 

 vitam methylenblue staining was also employed. 



In Dytiscus marginalis and Hydrophilus piceus 

 the blood consists of flocculent suspended material made up of 

 fine grannies, about 1 ju to "2 ^ in diameter, and of cells. These 

 latter are of two kinds — (1) phagocytes, and (2) small round- 

 cells.^ The number of cells counted varied from 120 to 500 

 per cubic millimetre in Dytiscus, and from 1030 to 4440 

 per cubic millimetre in Hydrophilus. 



The phagocytic cells are usually spindle-shaped when seen 

 ^ We have not observed blood-platelets in the plasma. 



