26 



BRYOZOA ECTOPROCTA. 



produced, and in this the plane of symmetry is denoted by the 

 position of the ectodermal furrow ; the larva is thus peach-shaped. 



For details of the structure of the Bugula larva we are indebted 

 chiefly to YIGELIUS (No. 39). Taking first the organs of the oral 

 surface, the sucker originates as an ectodermal invagination. In the 

 later stages the aperture of the invagination appears to close, at least 

 it was no longer visible. The upper part of the wall of the sucker 

 becomes invaginated into the cavity of the organ itself, thus greatly 

 compressing it. The whole rudiment in this way becomes cup-shaped. 

 In the anterior part of the oral surface a second ectodermal invagina- 

 tion appears ; this is found later between the cells of the corona, and 

 is known as the ectodermal furrow (e}. The ectodermal cells of this 



FIG. 9. A, larva of Bugula plumosa (after J. BARROIS). B, median section through a Bugula 

 larva (diagram constructed from figures by VIGELIUS). c, cells of the corona (a single cell 

 of this kind on each side) ; e, ectodermal furrow ; /, central tissue ; o, pyriform organ ; 

 p, mantle-cavity ; r, retractile disc ; s, sucker. 



region are elongated, columnar, glandular cells. In direct connection 

 with this modified glandular epithelium, there is a cell-mass which 

 extends towards the interior of the larva and ends posteriorly in 

 three processes. This is the pyriform organ (o). YIGELIUS is inclined 

 to regard the ectodermal furrow and the pyriform organ as a common 

 glandular complex derived from the ectoderm. 



The retractile disc (r) arises on the aboral surface as a simple 

 ectodermal thickening, the cells of which lengthen greatly and 

 become arranged in several layers. At the centre these elements 

 seem to be wanting, while at the periphery of the organ they show 

 a radial arrangement. This disc is protrusible beyond the surface of 



