THE MEDUSA OP MILLEPORA MURRAYI. 183 



with a sheath of ectoderm (Oon.) lining the cavity of the 

 ampulla; and this again is continuous with the superficial 

 ectoderm of the colony. 



At the margin of the umbrella both ectoderm and endoderm 

 are thicker than they are elsewhere, and the medusa presents 

 in consequence a thickened rim at its free border. There are 

 no radial or ring canals. In medusse at this stage no cavity 

 is apparent between the outer wall of the umbrella and the 

 ectoderm lining the ampulla. 



Above the codonostome (i.e. mouth of the umbrella) there 

 is an operculum (op.) of flattened ectoderm cells continuous 

 with the superficial ectoderm and the ectoderm lining the 

 cavity of the ampulla, which completely closes the gonangium. 



Different Forms of the Medusa. — The spermarium 

 varies immensely in size. - Sometimes it is simply a thickened 

 ring round the manubrium, sometimes it nearly fills the cavity 

 of the umbrella. In consequence perhaps of this variation in 

 the size of the spermarium, the appearance of the manubrium 

 varies. In fig. 10 the manubrium is a large well-developed 

 structure with a considerable lumen. In fig. 9, which repre- 

 sents a younger stage, there is no manubrium at all apparent, 

 but the spermarium simply rests on an irregular mass of vacuo- 

 lated endoderm cells. Many intermediate conditions between 

 these two extremes may be observed. Further, the condition 

 of the endoderm of the manubrium presents many variations. 

 In some cases the cell outlines are well marked, and the nuclei 

 regular in position and spherical in shape. In other cases the 

 endoderm is a loose vacuolated tissue in which no cell outlines 

 can be distinguished, and the nuclei are irregular in shape and 

 scattered through the spongy substance of the tissue. 



It is not my purpose to offer in this place any explanation 

 of these appearances. I wish merely to call attention to them 

 before passing on to other matters. 



Development of the Medusa. The medusa of Millepora 

 is a transformed zooid. It is not a zooid specially modified 

 from its first appearance to bear the spermarium, but an 

 ordinary zooid of the colony changed into a medusa after the 



6—2 



