GrEKEEA OF THE Il^ORTH AMERICAN PaLAEOZOIC BrYOZOA. ^469 



equivalent faces separated by the bourrelet, but now takes on 

 the form of a hat, of which the crown is formed by the oral 

 face, while the rims are formed by the bourrelet, which makes 

 a strong projection around the aboral face. The embryo has at 

 this time very much the structure described by Smitt as the struc- 

 ture of the embryo of Tuhularia serpens and CRisrA, and agrees 

 very well with the description of that author, of an embryo of the 

 form of a flat hat, with large rims, analogous in form to the em- 

 bryo of A^LCYONiUAf, described by Hinges. "1. It is composed 

 of a convex and a flat face, the latter surrounded by a projection 

 and showing by transparence at the center, an internal organ 

 formed by a round swelling of the outer layer, which is an open- 

 ing to the center of a cavity." The two organs which Smttt 

 describes as the rudiments of the tentacular crown, and the ad- 

 hesive organ, are probably the annular bourrelet, the evolution 

 of which has been described, and the digestive tube. 



At this stage the buccal opening, situated at the middle of 

 the oral face, is very distinct, and all this face, as well as the 

 superior part of the bourrelet, is provided with a continu .us 

 covering of long vibratile cilia. The cavity of the bourrelet {cm) 

 and the general cavity (cc) are still in direct communication as in 

 the preceding stage. 



All the rest of the development of the embryo consists of a 

 general shrinking of the form, with an elongation toward the 

 inferior part, whilst the bourrelet strongly projects beyond the 

 aboral face. In consequence of the general shrinkage, which 

 takes place rapidly, the embryo quickly changes from the dis- 

 coidal to the elongate form represented in fig. 14, Plate C. At 

 the same time the different organisms undergo modifications 

 corresponding to the change in form. 



The solid aboral face, hitherto almost flat, commences to project, 

 the projection becoming more prominent as the shrinkage pro- 

 ceeds, and eventually forming a round or somewhat elongate 

 mass (fig. 14, Plate C). While this round mass is forming, the 

 bourrelet projects more and more as a sort of mantle, and finally 

 the mass is almost entirely enveloped by the mantle, leaving only 

 a simple opening, which forms the communication with the in- 

 terior of the cavity circumscribed by the mantle. The figures 12 



