THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 



397 



as a mesonephros. The former is seldom functional, but the latter 

 is the functional organ in vertebrates up to and including the 

 Amphibia, as in frogs and salamanders. The metaiiephros is the 

 higher developed kidney as found in reptiles, birds, and mammals. 

 The ureter is the excretory duct which leads from the metanephric 

 kidney. The life history of these animals as individuals includes 

 successive stages as follows: the pronephros, the sole kidney for 

 a time; followed by the mesonephros which is the dominant func- 

 tional excretory organ when in its glory; and, finally, the develop- 

 ment of the metanephros with retrogression of the others. This is 

 an illustration of the Theory of Recapitulation which says that each 

 individual in its development lives through abbreviated stages of 

 the history of the development of the race. 



Excretion.^A certain result of the oxidation necessary for metab- 

 olism is the production of end-products which are not only of no 

 further use to the protoplasm but may be a distinct menace to the 

 welfare of the organism because of their toxic effects. The sub- 

 stances are usually dissolved and removed as a waste liquid or occa- 

 sionally as crystals by special parts of the body. 



In Protozoa this function is performed by general diffusion through 

 the plasma membrane and in many forms by the contractile vacuoles. 

 The quantity of water which passes through the protozoan in twenty- 

 four hours is several times the volume of the animal itself. Among 

 sponges and coelenterates diffusion of liquid wastes through the 

 general surfaces of the body to the surrounding water serves for 

 excretion. 



In an animal like the flatworm, planaria, excretion is accomplished 

 by a system of canals which begins in numerous capillary-sized 

 tubules whose blind ends are composed of individual cells called 

 flame cells. These flame cells are irregular in shape and each bears 

 a tuft of cilia extending into the end of the tubule. The flickering 

 movement of the cilia in the cell gives the appearance of a flame and 

 moves the accumulated excretion down the tubule. The waste liquid 

 of the surrounding tissues diffuses into this cell. The main excretory 

 ducts open to the surface of the body by excretory pores. This ar- 

 rangement is sometimes called a protonephridial system. 



The nephridial system is found in Annelida and has been studied 

 in connection with the earthworm. Here a coelomic cavity is present, 

 and a series of segmentaUy arranged pairs of coiled tubes or 



