215 



sonable to assume that the excretory function of the tubules has 

 been secondarily acquired and that the pronephros therefore first 

 functioned for the egestion of ek-phorocytes and gonocytes and that 

 later the excretory function was added to it. (see below.) 



Upon the evolution of the mesonephros followed by the meta- 

 nephros each of these organs in turn was better adapted than its pre- 

 decessor for excretory functions mainly from the fact that each was 

 nearer the point of exit to the exterior than the one before it. 



This factor, however, has no bearing upon the gonocytic and 

 egestive function and hence the primitive pronephric funnel persists 

 for the re-productive function (Mullerian funnel) ^■'^. 



If we suppose the primitive Chordate hermaphrodite ancestor to 

 have been protandrous^-^ then one would expect the male gonocytes, 

 ripening first, to be discharged by the mesonephric funnels, which are 

 nearer to the exterior and the female to retain their original pronephric 

 connection. 



Theseparation between the Wolffian and Mullerian ducts may 

 then be an expression of the separation between the ducts of the two 

 sexes in one individual rather than between the excretory and sexual 

 functions. 



3) The Polyzoa. The absence of any definite nephridia in the 

 marine colonial Polyzoa is a well-known morphological fact^^. This 

 is usually explained as an effect of the great reduction in size of the 

 individuals, an hypothesis wich one would hesitate to accept if any 

 other were tenable. Upon the recogaiton of the egestive function of 

 the nephrostomes, we have, however, a reasonable explanation of either 

 their atrophy or their not being evolved in the group in correlation to 

 the evolution of a definite coelomic cavity. 



We have seen that in a multicellular individual of the second or- 

 der (Spencer) there is a sacrifice of a single cell (or individual of the 

 first order) both for sexual (gonocytic) and egestive purposes (ekphorocy- 

 tic), — in precisely the same manner in individuals of the third order 

 (colonial hydroids and polyzoa) there should by natural laws be a "like 

 sacrifice of a whole individual of the second order (polype or zooid) in 



42 F. M. Balfour, Journ. Anat. and Physiol. X. 1875. 



« G. B. Howes, Linn. Soc. Journ. Vol.XXIII. p. 544. — A. T. Masterman, 

 Scottish Fish. Board Rep. 1895. 



^ In Loxosoina and Pedicellina have been described tubular excretory organs 

 with blind flame-cell terminations. (Harm er, Jo lie t.) «It is probable that the flame 

 cell termination, situated in the , primary' , body-cavity', is morphologically diff'erent 

 from the ciliated funnel which opens into the , secondary body-cavity' in Chaetopoda, 

 Mollusca, and Brachiopoda«. — S. F. Harmer, Q. J. M. S. Apr. 1885. — No ege- 

 stive function is ascribed in this Essay to flame-cell excretory organs. 



