344 ARTHUR WILLEY. 



phological grounds; but physiologically the atrophy of an old 

 intestine and the formation of a new one in animals which are 

 not pretended to have phylogenetically passed through an 

 intermediate parenchymatous condition^ is hard to conceive. 



Van Beneden and Julin got over the difficulty by suppos- 

 ing the presence^ in the ancestor of the Ascidians, of an 

 anterior '' collateral organ/' which served some function or 

 other concurrently with the existence of a terminal anus^ and 

 whichj as the old intestine atrophied, took on the definite 

 function of an intestine. In Amphioxus this collateral organ, 

 which has become the present intestine of the Ascidians, is 

 represented by its supposed homologue, the club-shaped 

 gland. 



I have previously (33) given reasons which seemed to me to 

 prove conclusively that whatever else it might be^ the club- 

 shaped gland of Amphioxus is a modified gill-slit pairing with 

 the first primary gill-slit proper. This being the case, it 

 becomes at once practically impossible to homologise it with 

 the intestine of the Ascidians. In the system of homologies 

 which I am seeking to establish I have already disposed of the 

 club-shaped gland as far as its morphological value is con- 

 cerned, so that we are driven to consider whether there is 

 really any sufficient ground for supposing that the tail of the 

 Ascidian tadpole represents the trunk of Amphioxus and con- 

 tains the rudiment of a primitive intestine. In fact, on con- 

 sideration it becomes evident that, taken alone, there is no 

 more to be said in favour of the view that the tail of Ascidians 

 represents the trunk of Amphioxus than there is for the view 

 that it is merely an organ of locomotion homologous with the 

 tail of Vertebrates and the post-anal region of Amphioxus. 



If the tail of the Ascidian tadpole was primitively segmented, 

 as most authors seem to think, or represents only one segment, 

 as Seeliger (31) thinks, there is in either case no more reason, 

 on this or that account, for regarding the endodermic cord in 

 the tail as representing a rudimentary intestine than for 

 regarding it as the remains of a post-anal gut. The latter was 

 the view which Balfour held (1, p. G34), and is most probably 



