322 
SIDNEY F. HARMER. 
Caldwell on Phoronis, and of Hatschek on Polygordius (17) 
and Echiurus (25), it appears possible that the ancestral 
Trochosphere-like form was provided with a well-developed 
body cavity, a structure represented also in some Molluscan 
Trochospheres. If this is the case, we must suppose that in 
most Trochospheres the formation of the body cavity has been 
postponed till after the commencement of the free existence. 
The Polyzoa are in this case probably descended from a 
Trochosphere-like organism, in which a postponement of the 
formation of the body cavity had become normal. It is possible 
that the Entoprocta have never advanced beyond this stage, 
whereas the Ectoprocta, in other respects more modified, 
have retained the habit of developing a body cavity. That 
this is really the interpretation of the body cavity of the 
Ectoprocta appears to me, however, in the highest degree 
doubtful, and in no known Ectoproctan ontogeny does a 
coelom develop in the embryo by an enterocoel formation or 
by any means which can be considered a modification of this 
process, as in Actinotr ocha. 
The larvae of the Ectoprocta are not provided, as far as is 
known, with a body cavity, a structure which appears first in 
the adult. In all probability the primary individual of the 
Ectoproctan colony is developed as a bud on the larva, and the 
wide difference in structure obtaining between the adult and 
the larva is quite comprehensible on this hypothesis. It 
seems to me that the body cavity of the Ectoprocta is a 
structure which is developed solely in relation with the adult 
condition, and that it is not directly comparable to that of 
other animals. The suggestion that the coelom of Ecto- 
procta is a space developed between gut and body wall for 
the purpose of permitting the retraction of the lophophore 
with its tentacles into the cavity of the zooecium appears to me 
not unreasonable. In the Gymnolsemata (see Yigelius, 
No. 46) the body cavity is a space traversed by irregular 
strands of connective tissue, and is as a general rule lined 
neither on the side of the gut nor of the body wall by an 
epithelial layer (Flustra). In the Phylactolsemata, how- 
