AMPHIOXUS AND AMMOCCETES. I/I 



It must be supposed that the original gill-slit, from 

 which the club-shaped gland is derived, acquired, for some 

 reason or other, a tubular form. 



A familiar precedent for gill-slits being drawn out into 

 elongated tubes, the effect of which is to separate the in- 

 ternal from the external opening by a long interval, is 

 presented by the hag-fish, Myxine. Myxine also shows us 

 that, in correlation with the canalisation of the gill-slits, 

 their external apertures may enter into new relations dif- 

 fering considerably from the primitive condition. As is 

 well known, the elongated tubular gill-clefts of Myxine do 

 not open separately to the exterior, but fuse together at 

 their distal extremities, so as to give rise to a longitudinal 

 duct on each side, which opens to the exterior some dis- 

 tance behind the gill-region. 



It is only on some such supposition as this namely, that 

 the external aperture of the gill-slit represented by the 

 club-shaped gland of Amphioxus has assumed new topo- 

 graphical relations in correlation with the canalisation of 

 the original slit that its position on the opposite (left) side 

 of the body to the internal opening of the gland is ren- 

 dered intelligible. The position of the internal opening 

 furnishes the criterion by which to judge of the primitive 

 relations of the original gill-slit. 



With the above point of view, therefore, we may signal- 

 ise the following facts to prove that the club-shaped gland 

 is the antimere of the first primary gill-slit. 



1. They arise simultaneously in the embryo as grooves 



in the ventral wall of the pharynx. 



2. They come to lie on opposite sides of the morphological 



median line the first gill-slit entirely so, and the 

 club-shaped gland in respect of its internal opening 

 into the pharynx. 



