PHYSIOLOGY OF THE INVERTEBRATA. 289 



the reason that there are strange facts to confront in deter- 

 mining the nature of an organ. For instance, the Malpighian 

 tubules of the Tnacctn are diverticula of the alimentary canal, 

 consequently they have been described as livers, and morpho- 

 logically they ouglit to have the function of a liver. But 

 when physiology, aided by chemical methods, steps in, we 

 find that these organs have solely a renal function. Is it 

 possible that the Malpighian tubules had originally the 

 function of a liver ? This is improbable, but it is well known 

 that an organ may lose its original function, and yet persist 

 because it is of use for some other purpose : one of these 

 predominate at one time, another at another, and the organ 

 undergo structural modification in consequence.* 



The variety of modifications or forms of the renal organs in 

 the Invertebrate maybe illustrated by the table on pp. 290-1. 



The table on p. 292 is a summary of the constituents 

 (present and absent) in the renal organs of the higher 

 hivertehrata. 



In the lower Tarcrtehrata the kidney performs other 

 functions besides that of a renal organ ; but in the higher 

 forms a special organ is set apart for that function, and it 

 resembles in many respects the Vertebrate kidney. 



On this point Prof. Huxley says : "In the Vertchrata, the 

 renal apparatus is constructed on the same principle [as the 

 renal organs of the Mollusca] .... The Vertebrate kidney 

 "is an extreme modification of an organ, the primitive type 

 of which is to be found in the organ of Bojanus in the 

 Mollusc, and in the segmental organ of the Annelid ; and, to 

 go still lower, in the water-vascular system of the Turbellarian. 

 And this, in its lowest form, is so similar to the more com- 

 plex conditions of the contractile vacuole of a Protozoon, that 

 it is hardly straining analogy too far to regard the latter 

 as the primary form of uropoietic as well as of internal 

 respiratory apparatus." 



* In the higher animals, for example, we have the formation of a lung 

 from a swimming bladder, and of the ear passage from a gill cleft. 



T 



