57 



ooecium in Flustid seciirijruns (PI. 1, figs. 5 ;i, h, c, PI. XIX, figs. 8 b — 8 n) whose 

 inner pari may be loolved upon as a transformed distal wall, is a helmet-shaped 

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

 |)ortion of the distal wall at a pointed angle, while its free, frontal edge passes 

 immediately over into the frontal membrane of the zoa'cium, together with which 

 it forms a fold, the od'cial fold. We may indicate the calcified bladder as the 

 endoott'cium, while an ectooo^cium is only represented by the portion of the 

 frontal 'membrane, which covers the distal pari of the endooo^cium. Between the 

 operculuiu of the zoa'cium 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 ou'cial membrane (PI. 1, fig. 5 a. 

 Pi. XIX, fig. 8j), which issues from the basal wall of the endooa-cium and forms 

 a complete separaling-wall across the oa-cium. While this was originally situated 

 close to the roof of the oa-cium, in an egg-bearing oa'cium it inclines the opposite 

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

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

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

 when it contracts, the oa-cial operculum is drawn inwards and thereby permits 

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

 of the zoa'cium issue two flat, sloping calcareous ribs which as a rule meet in 

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



The very earliest trace of an ooecium to be seen in Flustra seciirifrons 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 zocecium and rises from the attachment of the distal wall to the inner sur- 

 face of the frontal wall of the zoa'cium. PI. XIX, fig. 8 b shows a longitudinal 

 section through an early stage of an oa-cium, which is a little older than thai 

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

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

 which slill forms a straight line at this place. On the other baud, lig. 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 cjuite 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 



