POLYZOA. 249 



cells of the oral side, which are larger than the others, become divided 

 into two tiers, in such a manner as to form a prominence projecting into 

 the segmentation cavity. By the appearance of a lumen, in this promi- 

 nence it becomes converted into an archenteron, which communicates with 

 the exterior by a blastopore in the middle of the oral surface. The blasto- 

 pore becomes eventually closed. 



The archenteric sack of Repiachoff is clearly the same structure as 

 Barrois' four invagiriated cells of the oral face, their further history has 

 unfortunately not been followed out by Repiachoff. 



The free larva swims about for some time, and then fixes itself 

 and undergoes a metamorphosis ; but the exact course of this meta- 

 morphosis is still very imperfectly known. 



According to the latest statements of Barrois the attachment 

 takes place by the oral face 1 . The ciliated disc, which in the free 

 larva forms a kind of cup directed towards the aboral end, turns 

 in upon itself towards the oral face. It subsequently undergoes 

 degeneration and forms a nutritive or yolk-mass. The skin of the 

 larva after these changes gives rise to the ectocyst or cell of the 

 future polype. The future polype itself appears to originate, in part 

 at any rate, from the so-called dorsal organ 2 . 



The first distinct rudiment of the polype appears as a white body, 

 which gradually develops into the alimentary canal and lophophore. 

 While this is developing the ectocyst grows rapidly larger, and the 

 yolk in its interior separates from the walls and occupies a position in 

 the body cavity of the future polype, usually behind the developing 

 alimentary canal. According to Nitsche (No. 316) it is attached to a 

 protoplasmic cord (funiculus) which connects the fundus of the 

 stomach with the wall of the cell. It is probably (Nitsche, etc.) simply 

 employed as nutritive material, but, according to Barrois, becomes 

 converted into the muscles, especially the retractor muscles. 



Adopting the hypothesis already suggested in the case of the 

 Entoprocta the metamorphosis just described would seem to be a case 

 of budding accompanied by the destruction of the original larva. 



This view of the nature of the post-embryonic metamorphosis is appa- 

 rently that of Claparede and Saleiisky, and is supported by Claparede's 

 statement that the formation of the first polype ' resembles to a hair ' that 

 of the subsecaient buds. The mode of budding would, however, appear to 

 present certain peculiarities, in that the whole larval skin passes directly 

 into the bud, while from the rudimentary bud of the larva the lophophore 

 and alimentary tract only of the fixed polype are formed. 



Flustrella and Cyphonautes. The next group of larval forms is 

 that of which Cyphonautes is the best known type. The larvae 

 composing it at first sight appear to have but little in common with 



1 Barrois himself held the opposite view in his earlier memoir, and other observers 

 have done the same. 



2 The statements on this head are so unsatisfactory and contradictory that it does 

 not appear to me worth while quoting them here; even the latest accounts of Barrois, 

 which entirely contradict his early statements, can hardly be regarded as satisfactory. 



