1 66 Elementary Zoology. 



interesting to inquire why this is so. Possibly it has something 

 to do with the absence of any necessity for special organs of 

 respiration in thin-walled animals. 



The secretion of nitrogenous waste matters is (largely, at any 

 rate) an elaborate process of chemical manufacture, which 

 requires a special organ for the purpose. Once acquired, there 

 is no ii/«W2 reason why a different organ should be formed 

 for the same purpose. On the other hand, skin respiration 

 must be regarded as a primitive state of affairs, and special 

 respiratory organs were only needed as the organisms became 

 bulkier and consequently with thicker integuments. Hence 

 respiratory organs were produced at various periods and 

 separately in many groups of animals. Therefore there is no 

 reason why they should be of the same kind. 



In this connection it is interesting to note that many 

 animals with special organs of respiration still continue to 

 breathe partly by their skin. This is so with anodon ; and it is 

 well known that the skin of the frog is respiratory. In Helix 

 the mantle-fold has become so vascular, and so well adapted for 

 respiratory purposes, that the branchiae — present in allied 

 forms — have totally disappeared. 



