130 THE COCKROACH: 



In the living Insect the tubules remove urates from the blood 



o 



which bathes the viscera ; the salts are condensed and crystal- 

 lised in the epithelial cells, by whose dehiscence they pass into 

 the central canals of the tubules, and thence into the intestine. 

 The Malpighian tubules develop as diverticula from the 

 proctodooum, which is an invagination of the outer integument 

 and its morphological equivalent. They are, therefore, similar 

 in origin to urinary organs opening upon the surface of the 

 body and developed as invaginations of the integument, like 

 the " shell-glands " of lower Crustacea, and the " green glands " 

 of Decapod Crustacea. The segmental organs of Peripattis, 

 Annelids, and Vertebrates do not appear to be possible equiva- 

 lents of the excretory organs of Arthropods. They arise, not 

 as involutions, but as solid masses of mesoblastic tissue, or as 

 channels constricted off from the peritoneal cavity, and their 

 ducts have only a secondary connection with the outside of the 

 body or with the alimentary canal. 



Digestion of Insects. 



The investigation of the digestive processes in Insects is 

 work of extreme difficulty, and it is not surprising that much 

 yet remains to be discovered. Plateau has, however, succeeded 

 in solving some of the more important questions, which, before 

 his time, had been dealt with in an incomplete or otherwise 

 unsatisfactory way. The experiments of Basch, though now 

 superseded by Plateau's more trustworthy results, deserve 

 notice as first attempts to investigate the properties of the 

 digestive fluids of Insects. 



Basch set out with a conviction that where a chitinous lining 

 is present, the epithelium of the alimentary canal secretes chitin 

 only, and that proper digestive juices are only elaborated in 

 the chylific stomach, or in the salivary glands. The tests 

 applied by him seemed to show that the saliva, as well as the 

 contents of the oesophagus and crop, had an acid reaction, while 

 the contents of the chylific stomach were neutral at the begin- 

 ning of the tube and alkaline further down. From this he 

 concluded that the supposed deep-seated glands of the chylific 

 stomach secreted an alkaline fluid, which neutralised the acidity 



