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ooecium in Flustra seciirifrons (PL 1, figs. 5 a, b, c, PI. XIX, figs. 8b-8n) whose 

 inner part may be looked upon as a transformed distal wall, is a helmet-shaped 

 calcified bladder, the basal portion of which issues from the short and horizontal 

 portion of the distal wall at a pointed angle, while its free, frontal edge passes 

 immediately over into the frontal membrane of the zocecium, together with which 

 it forms a fold, the ooecial fold. We may indicate the calcified bladder as the 

 endoooecium, while an ectoooecium is only represented by the portion of the 

 frontal membrane, which covers the distal part of the endoooecium. Between the 

 operculum of the zocecium and the free rim of the ooecium we see a membranous 

 portion chitinized at the distal edge (PI. I, fig. 5 b), the ooecial operculum, which is 

 connected with another membranous region, the ocecial membrane (PI. 1, fig. 5 a, 

 PI. XIX, fig. 8 j), which issues from the basal wall of the endoooecium and forms 

 a complete separating-wall across the ooecium. While this was originally situated 

 close to the roof of the ooecium, in an egg-bearing ooecium it inclines the opposite 

 way, so that it forms the floor of the ocecial chamber (PI. 1, fig. 5 a, fig. 2 a). 

 In the angle between the ooecial operculum and the ooecial membrane is attached 

 a muscular chord which originates from the basal wall of the zocecium, and 

 when it contracts, the ocecial operculum is drawn inwards and thereby permits 

 the larva to escape. Finally, it may be mentioned that from the lateral walls 

 of the zocecium issue two flat, sloping calcareous ribs which as a rule meet in 

 a suture distally to the zooecial operculum (PI. 1, fig. 5 a, 5 b, 5 c, dw.). 



The very earliest trace of an ooecium to be seen in Flustra securifrons from 

 the surface of the colony, is a slightly inclined curved line, which at a distance 

 from the operculum nearly equal to its height unites the two lateral borders of 

 the zooecium and rises from the attachment of the distal wall to the inner sur- 

 face of the frontal wall of the zooecium. PI. XIX, fig. 8 b shows a longitudinal 

 section through an early stage of an ooecium, which is a little older than that 

 just mentioned and shows a distal wall, the basal portion of which is horizontal, 

 while the rest rises distally forming an angular arch and joins the frontal wall, 

 which still forms a straight line at this place. On the other hand, fig. 8 c shows 

 a small indentation proximally to the frontal end of the distal wall, and this 

 indentation increases in length or depth in the following figures 8 d, 8 e and 8 f, 

 the last of which represents a longitudinal section through a completed ooecium. 

 While it is quite easy to understand that Vigelius, who examined longitudinal 

 sections of decalcified colonies, considered the indentation mentioned to have risen 

 by an invagination of the frontal membrane, it is quite evident from the longi- 

 tudinal sections (8 b— 8 f) given here that the endoooecium is only formed by a 

 continued growth of the distal wall, which however at the same time undergoes 



