METAMORPHOSIS OF ACTINOTROCHA, 2038 
In the development of Actinotrocha, however, we find a 
metamorphosis involving profound and remarkable changes 
of structure, and yet produced by very simple processes of 
growth. The problem to be solved is much simplified and 
comparatively definite—reduced as it were to its lowest 
terms. While the contrast between the structure and 
habits of the larva and those of the adult is so striking as 
to show very clearly the general causes which have brought 
about the metamorphosis, it is possible, I believe, to get 
some idea of the exact way in which these causes may have 
operated in producing the peculiar transformation which the 
creature undergoes. 
In order to render these considerations intelligible, it will 
be necessary to give some description of the metamorphosis. 
The first part of this paper is therefore devoted to such a 
description, based upon my own observations and those of 
Metschnikoff. Although the account of the Russian em- 
bryologist is very complete and satisfactory, my own obser- 
vations are not without some value as supplying many 
hitherto unobserved details of the process, and from having 
been made upon two distinct species, neither of which can 
be satisfactorily identified with that studied by Metschnikoff. 
Certain discrepancies will be noted further on. I succeeded, 
moreover, in keeping the young Phoronis alive during several 
weeks—until they had, in fact, assumed the characteristic 
appearance of the full-grown animal. For the sake of com- 
pleteness, a few of Metschnikoff’s figures of the earlier stages 
have been introduced (figs. 1, 4.). 
Part I. 
It may be convenient to preface the following descrip- 
tion, with a short summary of the more important points. 
The larva has a somewhat elongated body with a series of 
long ciliated arms nearly encircling the body behind the 
mouth, and a narrow belt of long cilia just anterior to the 
anus, which is situated at the posterior end of the body. 
The mouth is on the ventral side near the anterior end, and 
is overarched by a very large hood-like expansion of the 
body wall, the preoral lobe. The hood is richly ciliated 
and serves to produce currents of water flowing towards the 
mouth, which bring with them particles of food. 
The only internal organ at first is the digestive canal, 
consisting of a distinct esophagus, a capacious stomach, 
and a longer or shorter intestine. In time, however, the 
ventral wall of the body thickens at a point in the middle 
