ON THE DEVELOPMENT OF TUBULIPORA. Ill 



are narrow aud pointed^ the younger stages of the bud are 

 longer than is usually the case in the Gymnolsemata ; and a 

 longitudinal section of the entire mass of protoplasmic struc- 

 tures appears as a sharply acute-angled triangle^ filling up the 

 narrow pointed tube which is at present the only representative 

 of the future zocecium. 



It may be assumed^ on the analogy of other cases,^ that the 

 inner layer of the vesicular bud is ectodermic, and the outer 

 layer mesodermic. The distal part of the bud gives rise to the 

 tentacle-sheath, into which the tentacles project; while the 

 proximal part develops into the remainder of the polypide, in 

 much the same manner as in other Ectoprocta. 



Immediately on the distal side of the two-layered polypide- 

 bud, which can be distinguished by the great readiness with 

 which it takes up colouring matters, there appears at an early 

 stage a cavity, lined by a thin layer of cells, which ultimately 

 gives rise to that part of the introvert which lies between the 

 calcareous "orifice'^ of the zooecium aud the '^diaphragm" 

 which forms the distal end of the tentacle-sheath. This cavity 

 is figured and described by many writers on the Ectoprocta. 

 It is shown by Nitsche (31, pi. xxxv, fig. 2) and by Prouho 

 (36, pi. xxiii, figs. 1 and 3), and it appears to be of general 

 occurrence throughout the Ectoprocta. Davenport (7, and 

 elsewhere) having termed the cavity of the tentacle-sheath the 

 " atrium," this space, with its wall, will be alluded to in 

 future as the '' vestibule,^' without thereby implying any 

 exact homology with the similarly named part of Polyzoon 

 larvae. I have previously figured the vestibule of a young 

 Cyclostome-bud (15, pi. xxii, fig. 1), although the space in 

 question was then erroneously described as the tentacle-sheath. 



In some cases the young vestibule appears as an invagina- 

 tion of the ectoderm of the terminal membrane, but it is often 

 difficult to obtain direct evidence on this point. 



The vestibule of the adult zooecium is usually a space of 

 considerable length, the diaphragm being a constriction be- 

 tween it aud the tentacle-sheath, as originally described by 

 ' Cf. especially Seeliger (38). 



