THE ANCESTRY OF THE CHORDATA. 567 
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 preoral 
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 
offers 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 
