THE MEDUSA OF MILLEPORA MURRAYI. 185 



of cases there is but a single cluster or aggregation (figs. 2, 3, 

 and 4). The largest and most fully developed of these lie at 

 the apex of the zooids (figs. 5, 6, and 7). 



The conclusions from these facts seem to be that the germ- 

 cells developing in the canals until they reach the stage corre- 

 sponding to the sperra-morula or spermosphere migrate towards 

 the zooids, fusing into aggregations as they do so. Having 

 reached the zooids they take up a position between the ecto- 

 derm and endoderm of their apices, and continue there the 

 later stages of their development. 



The spermospheres are most frequently found in the dacty- 

 lozooids, but in a few cases I have found them in gastrozooids 

 (fig. 3). They have probably no preference for either the one 

 form or the other ; but they are found more, frequently in the 

 dactylozooids, partly because these forms are more numerous, 

 and partly because the gastrozooids are usually more remote 

 from the larger coenosarcal canals. 



The spermarium having been formed at the apex of the 

 zooid certain noticeable changes take place. In the first place 

 by a thickening of the ectoderm the pore becomes narrowed 

 (figs. 5, 6, and 7). The tentacles become flattened out, and 

 the nematocysts disappear. The spermarium sinks into a cup- 

 shaped receptacle on the summit of the zooid, and the endo- 

 derm of the edge of the cup grows out, pushing before it the 

 ectoderm. 



These changes are represented in the two figs. 6 and 7 In 

 the next stage the cup-shaped receptacle of the spermarium 

 has grown out into a bell-shaped structure (fig. 8). The sper- 

 marium is much larger in size, and the pore is completely 

 closed by ectoderm. In the later stages (figs. 9, 10, and 11) 

 the following changes may be noted. The operculum is formed, 

 shutting off all access from the cavity of the gonangium to the 

 exterior. The walls of the bell-shaped outgrowth become con- 

 siderably attenuated, and lie close against the ectodermic wall 

 of the ampulla. The manubrium is formed probably by a 

 regeneration of the endodermic tissue and its growth into the 

 centre of the spermarium. 



