C INTRODUCTION. 



distinguishable. As Vogt has remarked, "it is difficult 

 to imagine that all these organs thus definitely constituted 

 are to disappear that a new and similar set may be 

 evolved." Van Beneden's observations are especially 

 precise ; and I had always regarded them as conclusive. 

 Barrois, however, is of opinion that the cycle of develop- 

 ment is the same for Pedicellina as for the Cheilostomes ; 

 and he has recorded observations which, so far as they go, 

 seem to sustain this view. Subject to further investiga- 

 tion, his conclusion is that histolysis takes place in Pedi- 

 cellina as in the other Polyzoa, and that the passage from 

 the larva to the adult is less direct and simple than has 

 been usually supposed. We must await the results of a 

 fuller study of the question. 



Putting out of view the case of the Entoprocta, it 

 seems probable that the larva of the Polyzoa after fixa- 

 tion always undergoes a partial histolysis, affecting espe- 

 cially the provisional structures belonging to its free life, 

 and passes by gradual metamorphosis into the perfect 

 animal*. 



As the result of his extended researches into the embry- 

 ology of the Polyzoa, Barrois reduces the various larval 

 forms to a single primitive type, consisting of&ffastrula with 



* Joliet, indeed, has suggested a different view. To him the larva may 

 be considered as composed of a zooecium and a zooid (the homologue of 

 the polypide), both of them modified with reference to the necessities of a 

 free life the zooecium more highly organized than in the adult, the poly- 

 pide rudimentary. The histolysis of the larva he compares to that of the 

 polypide in the ordinary cell. When the former is on the point of fixing 

 itself, the zooid which it contains dwindles away just as the latter does, and 

 disappears. The larva of Pedicellina is strictly comparable to that of the 

 other Polyzoa ; it consists of a zooecium and a contained zooid ; but as the 

 ordinary polypide in this case does not undergo histolysis, neither does the 

 larval zooid ; it passes into the adult form, which it very closely resembles 

 (' Bryozoaires d. cotes de France,' pp. 85, 80). Of course, if the observa- 

 tions of Barrois are confirmed, they exclude this interpretation. For my- 

 self, so far as the Entoprocta are concerned, I await the results of further 

 investigation. 



