262 COELENTERATA HYDROZOA chap. 



undoubtedly 3fillepora occur in the raised coral reefs of relatively 

 recent date, but do not extend back into Tertiary times. There 

 seems to be no doubt, therefore, that the genus is of comparatively 

 recent origin. Among the extinct fossils the genus that comes 

 nearest to it is Axopora from the Eocene of France, but this genus 

 differs from MiUepora in having monomorphic, not dimorphic, 

 pores, and in the presence of a minute spine or columella in the 

 centre of each tube. The resemblances are to be observed in the 

 general disposition of the canal system and of the tabulation. 

 Whether Axopora is or is not a true Milleporine, however, cannot 

 at present be determined, but it is the only extinct coral that 

 merits consideration in this place. 



Order III. Gymnoblastea — Anthomedusae. 



This order was formerly united with the Calyptoblastea to form 

 the order Hydromedusae, but the differences between the two are 

 sufficiently pronovmced to merit their treatment as distinct orders. 



In many of the Gymnoblastea the sexual cells are borne by 

 free Medusae, which may be recognised as the Medusae of 

 Gymnoblastea by the possession of certain distinct characters. 

 The name given to such Medusae, whether their hydrosome stage 

 is known or not, is Anthomedusae. The Gymnoblastea are 

 solitary or colonial Hydrozoa, in which the free (oral) extremity 

 of the zooids, including the crown of tentacles, is not protected 

 by a skeletal cup. The sexual cells may be borne by free 

 Anthomedusae, or by more or less degenerate Anthomedusae 

 that are never detached from the parent hydrosome. The 

 Anthomedusae are small or minute Medusae provided with a 

 velum, with the ovaries or sperm-sacs borne by the manubrium 

 and with sense-organs in the form of ocelli or pigment-spots 

 situated on the margin of the umbrella. 



The solitary Gymnoblastea present so many important diHer- 

 ences in anatomical structure that they cannot be united in a 

 single family. They are usually fixed to some solid object by 

 root-like processes from the aboral extremity, the " hydrorhiza," 

 or are partly embedded in the sand (Coiymorpha), into which 

 long filamentous processes project for the support of the zooid. 

 The remarkable species Hyj)ohjtus p)eregrinv.s ^ from Wood's Holl, 

 1 Quart. Journ. Micr. Sci. xlii. 1899, p. 341. 



