PHYSIOLOGY OF THE INVERTEBRATA. 267 



seen to consist of a connective tissue layer, a delicate 

 " tracheal tube," a basement membrane, and an epithelial 

 layer of comparatively large nucleated cells (Fig. 50). The 

 internal cavity of one of these tubules is very irregular, as is 

 seen by examining various parts of it in a transverse 

 section. 



The uric acid contained in these tubules can be extracted 

 by boiling a large number of them in water, filtering, and 

 then evaporating the filtrate to dryness. The residue is 

 treated with alcohol, filtered, and the residue so formed is 

 dissolved in boiling^ water to which acetic acid is added. 

 After standing for several hours, crystals of uric acid 

 (C.H^N/)3) are deposited. These crystals are readily con- 

 verted into murexide. 



Then, again, if a fresh Malpighian tubule is placed upon a 

 slide under the microscope, and crushed, a drop of dilute 

 acetic acid added, and the whole covered by a cover-glass, 

 rhombic and other crystalline forms are deposited. These 

 crystals are also readily converted into murexide by the 

 action of nitric acid and ammonia. 



No other substance besides uric acid could be detected in 

 the Malpighian tubules of Libclhda dcprcssa. 



From the above-mentioned reactions it is evident that the 

 ]\Ialpighian tubules of the Insccta are physiologically true 

 renal organs. 



As already mentioned some zoologists of the older school 

 stated that these appendages of the alimentary canal repre- 

 sented the " liver," and this statement has been recently 

 revived by Dr. B. T. Lowne in his work on CalUphora. But 

 the ^lalpighian tubules of the Diptcra (including CalHphora) 

 readily yield uric acid when the proper tests are skilfully 

 applied ; and they do not contain the least trace of biliary 

 acids, glycogen, or even ferments. 



The ^lalpighian tubules of the Insccta are undoubtedly true 

 kidneys, although they are developed from the alimentary 

 canal. 



