134 BULLETIN OF THE 



The tentacles of Phylactolaemata may be distributed into two groups. 

 The first includes those which arise from the circumoral branch of the 

 ring canal. The ring canal, from which they spring, begins to be formed 

 at nearly the same time as the lophophoric arms. These tentacles are 

 undoubtedly homologous with those of the same region in Endoprocta 

 and Gymnola3mata. The second group of tentacles includes those which 

 are borne upon the lophophore arms and upon the supraganglionic ring 

 canal. Are these comparable with the posterior tentacles of Gymnoke- 

 mata 1 I believe they are, and for the following reasons. Nitsche's 

 reason for supposing that they are not is unsatisfactory, since, if we re- 

 gard the lophophore arms as mere upward folds of the wall of the ring 

 canal, we should expect to have the tentacles arise later than the arms. 

 The fact that the tentacles of the lophophore arm arise much later than 

 those of the circumoral region is what we should expect, since the pos- 

 terior tentacles arise later than the circumoral ones in both Endoprocta 

 and Gymnolsemata, — a criticism which Hatschek ('77, p. 541) has 

 already applied. In direct support of my belief are the facts, (1) that 

 the ring canal is continuous along two sides of the lophophore arms, 

 which would be the case if they were mere upward folds of the wall of 

 the ring canal ; (2) the structure of the tentacles is the same as that 

 of the oral ones, and the relation of their intertentacular septse to the 

 ring canal of the arms is the same as that of the septse of the oral ten- 

 tacles to their ring canal, as Kraepelin ('87, pp. 55, 56) has shown. If 

 both circumoral and lophophoric tentacles find their homologues in 

 Gymnolsemata, we have only to conceive of an elongation of the postero- 

 lateral angles of the lophophore of Gymnoleemata, after the forward 

 movement of the posterior tentacles, to effect the condition which is 

 found in Phylactolaemata. 



The significance of the fusion of the lophophore arms is difficult to 

 determine. I had thought it might be possible to find a phylogenetic 

 explanation for it, by regarding the unfused tips of the arms in Crista- 

 tella as homologous with the short arms of Fredericella. In studying 

 Plumatella, however, where the length of the lophophore arms is inter- 

 mediate between that of Cristatella and Fredericella, I have been able 

 to find no trace of this fusion. It does exist, however, in Pectinatella. 

 I have had no material of Lophopus, upon which it is important to study 

 this point. The evidence so far seems to indicate that this fusion of the 

 arms during the period of their development is a secondarily acquired 

 adaptation to some condition concerning the nature of which I am 

 ignorant. 



