TECHNICAL NOTES AND BIBLIOGRAPHY 259 



Glomerular development in Cyclorana is discussed by 

 Dawson {85}. Water economy in this and related forms 

 is discussed by Buxton (see Chapter XI) . 



This is as good a place as any to discuss the function 

 of a urinary bladder. Whatever usefulness may be read 

 into this organ in the higher animals, it is rather sur- 

 prising that a urinary bladder is fully developed in all 

 the Amphibia and in most fishes. No bladder is present 

 in the elasmobranchs, in which the ureters open into a 

 common urogenital sinus or directly into the cloaca. 

 Neither is a bladder present in the cyclostomes or the 

 Dipnoi; but one is present in the surviving ganoids and 

 generally in the teleosts, in which the archinephric ducts 

 unite and expand into a single urinary sinus {49, p. 407}. 

 The bladder in the fishes is, however, derived from 

 mesoderm and is not homologous with the cloacal blad- 

 der of the tetrapods. No bladder is present in the rep- 

 tiles, except for the hzards and turtles, or in the birds 

 or the two primitive egg-laying mammals, the platypus 

 and spiny ant-eater. 



In the Amphibia one can see how a large virinary 

 bladder might afford a small supply of dilute fluid to be 

 reabsorbed when the animal is threatened with desicca- 

 tion, but that this is the explanation of its evolution must 

 be set aside in view of the presence of the organ in the 

 primitive fresh- water fishes, where there is no need for 

 such reabsorption, as well as in many highly specialized 

 marine fishes where water reabsorption would be impos- 

 sible because the urine is already maximally concen- 

 trated. Most marine fishes have a very large bladder 

 and it is frequently foimd to be full of urine. What seems 

 a more cogent suggestion is that the urine, being perhaps 

 detectable by enemies, could leave an unbroken trail 

 for pursuit were it not retained and discharged at inter- 

 vals. The goosefish, Lophius, for example, may carry 

 around with it as much as 5 per cent of its body weight 

 as urine, though at the normal rate of urine formation 



