212 EDMUND B, WILSON. 
stage a deep infolding of the body wall grows in between 
the mouth and anus, so that the previously concealed flexure 
becomes apparent. It may be added that the flexure subse- 
quently straightens out and the worm assumes the ordinary 
form. This mode of development is the converse of what I 
take to have been the ancestral history of Phoronis. 
These facts are not very full, yet they are enough to 
demonstrate the possibility of such a process of transformation 
as that which has been assumed to have taken place among 
the Gephyrea. The case would be stronger if there were in 
existence any forms intermediate between the two series of 
Gephyrean forms. But it must be admitted that no such forms 
are now known. In certain species, it is true, the mouth and 
anus are much more widely separated than in Phoronis. 
These forms show no external evidences of any flexure of 
the body; and if they were really intermediate forms their 
structure would furnish a strong argument against the 
flexure hypothesis. But it must be noted that in many of 
these forms the anterior or oral part of the body walls may 
be invaginated to form a sort of false esophagus with the 
true mouth at its bottom. And it seems probable that the 
considerable interval between the mouth and anus exists 
simply in order to permit such invagination to take place 
without involving the latter opening. If this surmise be 
correct, the forms in question have really been derived from 
others which had the mouth and anus closely approximated, 
and therefore they are not intermediate forms at all, and 
present no difficulties in the way of the view which has been 
advanced. 
Assuming, then, that the peculiar structure of Phoronis 
has resulted from the flexure of a primitively straight form, 
we may proceed to a consideration of the metamorphosis as 
viewed in this light. In the first place, what general causes 
have rendered this sudden and violent metamorphosis service- 
able to the animal? The answer lies in the circumstance that 
the animal is subjected, during its larval existence, to con- 
ditions of life utterly unlike those by which it is surrounded 
when adult. At first pelagic and actively free-swimming, 
it subsequently assumes a purely sedentary mode of life, 
inhabiting a fixed tube which it never leaves, and in which 
movement is limited to comparatively sluggish extensions 
and contractions of the body. In passing from one existence 
to the other the animal must undergo a complete revolution 
in its mode of life; as a matter of course, this profound 
change can be possible only if accompanied by a correspond- 
ing modification of structure. In fact, the larva and adult 
