58 BULLETIN OF THE 



one time ; but as he has not attended particularly to this point, this can 

 hardly be said to militate against my view. 



There is, however, in my opinion, a more important error in Seeliger's 

 description of the origin of the alimentary tract, — an error into which 

 Nitsche ('71, p. 457) also fell. As in Phylactolsemata and Paludicella, 

 so also in marine Bryozoa in general, so far as I have studied them, the 

 posterior and anterior parts of the alimentary ti'act are formed indepen- 

 dently, and their cavities coalesce only secondarily. The constriction 

 which separates the lumen of the bud into a cavity nearer, " vorder," and 

 one more remote from the body wall, " basal," does not separate off the 

 whole alimentary tract from the atrium. Neither does that constriction 

 result in the formation of a space opening into the cavity nearer the 

 body wall, " Vordertheil," at an upper [distal] point (anus) and lower 

 [proximal] point (mouth). Thus if one examines a complete series of 

 sections through a polypide even of so late a stage as Figure 92 

 (Plate X.), one finds that, while there is an open connection between the 

 anal end of the alimentary tract and the atrium, the oral end is at all 

 points sharply separated from the cavity above by a double-layered wall 

 of cells, as is shown in Figure 92, between oe. and ga. Such a condition, 

 moreover, has been found by Barrois ('86, pp. 73-76) in the primary 

 polypide of Lepralia, and by Prouho, as just stated, in the primary 

 polypide of Flustrella. 



Origin and Development of the Ring Canal and Tentacles. — Nitsche 

 ('71, p. 430) first described in Flustra a ring canal surrounding the 

 mouth-opening and lying at the base of the tentacles, but did not refer 

 to the origin of it. Seeliger ('90, p. 588) describes it in a young pol- 

 ypide of Bugula, as derived from the mesodermal layer. 



My own sections also show that it arises on each side of the oesopha- 

 gus as a groove lined by mesoderm (Plate X. Fig. 92, right). This 

 canal, which is shown cut along its course in Plate IX. Fig. 82, can. 

 crc, is not wholly separated from the body cavity, but communicates 

 with it below the brain. This communication occurs in the section 

 below that shown in Figure 82, near the point can. crc. This ring 

 canal at an earlier stage is shown in Figure 87. It has not yet been 

 formed backwards nearly so far as the brain ; anteriorly the section has 

 traversed the tentacles under which it runs. The canal is also shown 

 cut across in Figure 86 at the base of a tentacle, with whose lumen its 

 cavity is directly continuous. 



The formation of the tentacles is closely connected with that of the 

 ring canal, from the upper wall of which they arise. Since the upper 



