THE MEDUSA OF MILLEPORA MURRAYI. 403 



4. The medusae are formed by a metamorphosis of an 

 ordinary zooid; in the majority of cases dactylozooids, but 

 in others gastrozooids. 



5. The sperm- cells originate in the ectoderm of the coeno- 

 sarc and wander into the ectoderm of the zooids, where they 

 fuse into aggregations to form a spermarium. 



6. The young spermarium is formed at the distal extremity 

 of the dactylozooid, and when it has reached a certain size 

 it causes a retrograde metamorphosis of the tissues. The 

 tentacles flatten out and disappear, and the zooid loses all 

 its characteristic features. 



7. A cup-shaped outgrowth next appears which forms the 

 umbrella of the medusa, and subsequently a conical growth 

 of the endoderm penetrates into the substance of the sper- 

 marium and forms the manubrium. 



8. The male gonophores of Distichopora occur in groups of 

 two or three in each ampulla in different stages of develop- 

 ment. The gonad is supported by a small cup-shaped 

 trophodisc, and enclosed in a double sac of ectoderm and 

 endoderm. At the distal pole of the ripe gonophore there is 

 a short seminal duct. 



9. The male gonophore of AUopora differs from that of 

 Distichopora, in the fact that it is provided with a club-shaped 

 endodermal manubrium or spadix. 



10. The female gonophore of Distichopora resembles that 

 of AUopora described in a previous paper; but the folds of 

 the trophodisc are not so complicated. 



11. The gonophores of the Hydrocorallinse are not 

 degenerate medusae. 



