2) 
MUSEUM OF COMPARATIVE ZOOLOGY. 31 
their anatomy, have been struck by their resemblance — especially in 
the possession of a body cavity and of a tentacular corona — to the 
Gephyrea, and particularly to Phoronis. Those, on the contrary, who 
have devoted their studies chiefly to the Endoprocta, and especially to the 
development of that group, have urged the second view. The third view 
seeks to reconcile the two conflicting theories. I favor the second of the 
views given above because of certain considerations which follow. 
I propose first to show the untenableness of the third view. It would 
then be nearly sufficient, in deciding between the two remaining views, 
to show that the Endoprocta are the more primitive group of Bryozoa ; 
but in addition to this, I shall offer positive evidence of derivation of the 
Bryozoa from the lower worms. 
The chief argument for the diphyletic origin of Bryozoa rests on 
these three important differences between Ectoprocta and Endoprocta : 
(1) that of the tentacular corona, which includes within it the anus in 
one case, and leaves it outside in the other ; (2) that of the body cavity, 
which is absent in one case and present in the other ; and (3) that of 
the kidney, which is a pronephridium in Endoprocta and (it is alleged) 
a metanephridium in Ectoprocta. 
I have, in an earlier paper (91%, p. 103), shown that the difference in 
relations of the anus to tentacular corona is completely and satisfactorily 
explained by the study of the development of the polypide, in which the 
closure of the tentacular corona between mouth and anus is effected only 
at a relatively late stage. 
Concerning the second of these differences, Ehlers (’90, pp. 152, 154) 
has already well argued that it cannot be so fundamental, since other un- 
questionably closely allied groups (e. g. Hirudinea and Cheetopoda) differ 
similarly. Moreover, the difference between the “ body cavity ” of Gym- 
nolemata and Endoprocta is one of degree, not of kind, for in both cases 
we have to do with parenchymatous tissue more or less completely filling 
the primary body cavity. The existence of spaces in the midst of the 
parenchyme of Gymnolemata may be accounted for (following Harmer, 
’85, p. 64, see also Lang, ’88, p. 77) on the physiological ground of the 
necessity of a space into which the polypides can retract. In Phylac- 
toleemata this parenchyme has become, in part, a very definite “ coelomic 
epithelium,” although, as I have pointed out (’90, p. 128), showing 
traces of its parenchymatous origin. 
Upon the alleged differences in the kidney, no argument can be based, 
simply because the existence of an excretory tubule in Ectoprocta is 
very uncertain, being at present not even probable. 
VOL. XXIV. — NO. 1. 3 
