ANURIDA. 401 



concretion is made up of a covering coat investing a 

 central crystalline ( ?) mass. This is seen when nitric acid 

 is made to act very slowly, when the central contents 

 dissolve first and subsequently the coat vanishes also. 

 Sommer believes that they are composed of calcium 

 carbonate, together with an organic basis. 



\Villem has also studied these excretory concretions, 

 and he used for the purpose Sminthurus fuscus. He 

 remarks (27) that they have a concentric structure, 

 and that they are insoluble in water, alcohol, ether, or 

 chloroform, but are dissolved by acids. By means of 

 appropriate tests, he came to the conclusion that they are 

 formed of a neutral urate of sodium. It is probable that 

 those of Tomocerus are of a similar composition, rather 

 than of calcium carbonate, and Sommer mentions that he 

 did not apply any uric acid tests. 



A remarkable feature regarding the excretory 

 function in Collembola is the absence of any ducts by 

 means of which the products can be eliminated. The 

 concretions consequently increase in size according to the 

 age of the animal, and render it impossible for the lives of 

 these insects to be prolonged for any lengthy period. An 

 analogous case is met with in the Ascidians among the 

 Chordata. The " renal vesicles " of these animals 

 similarly have no means of getting rid of the excretory 

 material stored up within them, and which increases 

 greatly in quantity as the individuals progress in age. 



It cannot be decided, in the light of existing 

 knowledge, whether the fat-body of the Collembola is 

 homologous with that of other insects. In the case of 

 the latter, authorities are far from being in agreement as 

 to its mode of development, and nothing is known of the 

 development of the fat-body in the Collembola. Both 

 Sommer and Willem claim that among the latter 



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