224 ARTHUR WILLEY. 



shaped gland is a rudimentary structure which formerly served 



as an intestine^ or whether it is what van Beneden and Julin 

 would call a primitive collateral organ, which in the Ascidians 

 has come to function as an intestine. A great many con- 

 siderations render it only reasonable to suppose that the intes- 

 tine and anus of Amphioxus are primitive, and hence the 

 club-shaped gland must be regarded as an ancestral organ 

 occurring in company with a posterior anus. 



Assuming, then, that the ancestor of Amphioxus possessed a 

 collateral organ of unknown function situated behind the 

 primitive median endostyle, with internal and external orifices 

 as described, it is difficult to conceive how it can have become 

 evolved, except by the modification of some pre-existing organ. 

 The spectacle of an unpaired hypoblastic gland opening to the 

 exterior on the one hand, and into the alimentary canal on the 

 other, without having a morphological equivalent in the whole 

 animal kingdom, is certainly too much. 



There are thus two points to settle : 



1. What was the nature of the pre-existing organ referred 

 to above ? 



2. Was it paired or unpaired ? 



It may be remembered that the intra-buceal orifice of the 

 club-shaped gland, which was described and figured for the 

 first time in the paper by Professor Lankester and myself quoted 

 several times above (8), is not present in the very young larvae, 

 but forms later. This is probably another instance of a slight 

 and unimportant retardation in development. 



Before becoming acquainted with the views of van Beneden and 

 Julin I had come to the conclusion, on grounds which seemed 

 sufficient, that the club-shaped gland was a modified gill-slit; 

 and, as I am aware of no other attempts to account for the mor- 

 phology of the gland, I will give reasons for so regarding it. 



In the second place, there are reasons for supposing that it 

 was originally one of a pair of gill-slits, and further, that the 

 original pair actually exists in the larva of Amphioxus — the 

 members of the pair being the club-shaped gland and the first 

 primary gill-slit. 



