AMPIIIOXUS AND AMMOCOiTES. IJl 



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, Myxinc. 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. 



