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 sufiicient, 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 diff'erences 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 Chsetopoda) differ 

 similarly. Moreover, the difference between the " body cavity " of Gym- 

 nolaemata 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 Gymnolsemata 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- 

 tolsemata 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 



