28 



BRYOZOA ECTOPROCTA. 



^^^-s 



the ectoderm which previously formed the lining of the circular 

 furrow (mantle-cavity, p\ and which in the larva are distinguished 

 by the great depth of their cells (Fig. 10, A). The mantle-cavity 

 (p) is in this way completely obliterated, the circular fold that 

 bordered it externally, and was called the mantle, bending over 

 downwards (Fig. 10, B) ; its inner thus becomes its outer surface, 

 and gives rise to the body-wall of the primary zooecium. The 

 corona at the same time bends round or unfolds (Fig. 10, B). The 

 lower ends of the coronal cells (c) retain their position, while the 

 upper ends first shift outward (Fig. 10, A) and then downward 

 (Fig. 10, B), so that each coronal cell at 

 the end of this process has rotated through 

 an angle at first of 90° and finally of 180° 

 (Fig. 1 1 c). We thus have a stage to which 

 Barrois has given the name of the umbrella- 

 shaped stage (Fig. 10, B). The upper 

 surface is formed by the future body-wall, 

 the lower by the downwardly rotated 

 coronal cells. The edges of the umbrella 

 become applied to the adhesive plate of 

 the sucker (s) and fuse with it, the body- 

 wall thus completing itself (Fig. 11). 

 The displaced coronal cells at the same 

 time fuse with the upper surface of the 

 sucker, and in this way a circular cavity 

 arises {vestibulum, Barrois, Fig. 11 v), the 

 walls of which soon become completely 

 detached from the body-wall and undergo 

 degeneration. This is the fate not only of the corona, but of the 

 ectodermal furrow, the pyriform organ, and (where such is present) 

 the larval intestine. There is then found inside the sac-like larva a 

 cell-mass derived from the degeneration of the most important larval 

 organs; this unites with the mass that resulted from the degeneration 

 of the central tissue, and forms the so-called broivn body. 



The rudiment of the body- wall of the primary zooecium is framed 

 in the way just described. Its surface soon becomes covered with 

 a chitinous secretion (often impregnated with lime salts) ; this is 

 the ectocyst. 



During these ontogenetic processes, other changes can be seen to 

 occur at the aboral pole which result in the development of the 

 polypide of tin- primary individual. The retractile disc that lies 



Fio. 11. — Longitudinal section 

 of attached larva of Bugula 

 jlabdlata (after J. Barrois). 

 c, coronal cells ; r, rudiment 

 of the polypide (which has 

 arisen by the invagination of 

 the retractile organ) ; s, lower 

 thickened surface of the 

 evaginated sucker (adhesive 

 plate) ; v, vestibulum. 



