THE ANCESTRY OF THE CHORDATA. 99 



chamber are to be regarded as secondary perforations. 

 Whether this ingenious theory be adopted or not^ the fact 

 remains that Appendicularia is almost certainly a very primi- 

 tive Tunicate, and also that the arrangement of the pharyngeal 

 perforations of other Ascidians makes it unlikely that they are 

 homologous with the gill-slits of higher forms. 



The increase in size of the tail, which would speedily 

 follow the first use of the backward directed notochord as a 

 swimming organ is not difficult to understand. In connection 

 with the increase of the tail the curvature of the gut would 

 also be intelligible. From atrophy of the preeoral lobe in cor- 

 relation with the future sessile habit^ coupled with increase of 

 the lower lip to bear the suckers, the relations of the neural 

 pore to the mouth would result. The gland of the piseoral 

 lobe would then, as before described, be placed below the 

 nerve-ganglion and open into the pharynx. 



It has been remarked by Seeliger (' Jen. Zeit.,' 1885) that 

 the body of the Ascidian tadpole appears to consist of one head 

 and two trunk segments. It may be observed that though the 

 reasons for this belief are not very obvious, this view, if correct, 

 would coincide with the possibility of its descent from such a 

 larva as Balanoglossus, Stage G, which also possesses one head 

 and two trunk segments. 



However the various points that have been raised in the 

 preceding paragraph may be decided, it has seemed necessary 

 to point out what conclusion with regard to the structure of 

 Ascidians may be drawn from the development of Balano- 

 glossus. That these are so meagre is to be regretted ; the only 

 tangible point appears to be the confirmatory evidence that it 

 oflfers to the view that the atrial folds of Tunicata are not 

 homologous with those of Amphioxus. 



In this way only can the absence of mesoblastic repetitions 

 in Tunicates be accounted for. Their development gives no 

 support to the view that their ancestors possessed repetitions 

 of this kind. 



Of the Enteropneusta. — That the Enteropneusta might 

 possibly have had an ancestor in an animal possessing the 



