HYDROZOAN CORALS 149 



known, for instance, whether they are produced periodically 

 or spasmodically, or whether their production is due to 

 environmental conditions that affect all the Millepores of the 

 reef at the same time, and it is also not known at what size 

 or age they first begin to produce medusae. 



All that can be said at present is that when the collections 

 of corals in museums are examined very few specimens are 

 found that exhibit the ampullae in which the medusae are 

 lodged, and this suggests that the phenomenon occurs at 

 long intervals of time and does not last long. 



The medusa consists of an umbrella and a short stumpy 

 manubrium, which is, in some cases, provided with a mouth 

 in the female medusae but never in the male (Fig. 67, Med.). 

 The umbrella is extremely thin, and bears neither radial nor 

 ring canals. Close to its margin there are four or five knobs, 

 each one consisting of a battery of nematocysts, but apart 

 from this there are no tentacles. In the ripe female medusae 

 four or five relatively large yolk-laden eggs are borne by the 

 manubrium. In the ripe male medusae the testis is in 

 the form of a ring round the manubrium. The size of the 

 medusa in both sexes is about 0-4 mm. 



It is very improbable that the medusae have a long free- 

 swimming life, and Mr. Duerden has observed that the 

 female medusae discharge their eggs within five or six hours 

 of their liberation. 



Although Millepora occupies such an isolated position in 

 the animal kingdom, for it has really no near relation among 

 the corals, there is no evidence that it had made its appear- 

 ance on the reefs even as late as the Tertiary geological period. 

 It is true that a number of corals which have been given the 

 name Millepora by various authors are found in the Tertiary 

 and even older rocks, but a careful examination of these 

 fossils shows that not one of them possesses the very dis- 

 tinctive characters of the corallum of Millepora. 



The only fossil coral that approaches Millepora in struc- 

 ture is the genus Axopora from the Eocene of France, but 

 this coral has monomorphic pores and each pore bears in its 

 centre a minute spine or columella. 



Millepora is a common constituent of the coral reefs of 



