332 DE. S. J. HICKSON — EEVISION OF THE 



their polypes. Since Ehrenberg's time new species have been named Bhizoxenia with 

 perfectly retractile polypes, so that the original character of the genus has been lost. 

 Thus von Koch (17) in a recent paper says that Bhizoxenia rosea (von Koch) differs from 

 Clavularia in the fact that the tentacles are partially invaginated in retraction as in 

 Coralliimi ruhrum ; but this is not a character that can be raised to the value of a specific 

 distinction. I have myself noticed in some species of Clavularia that the base of the 

 tentacle is, to a certain extent, invaginated, and I have always considered that this 

 feature depends upon the presence or absence of a dense deposit of spicules at the basal 

 part of the tentacles. In those Clavularias with very thick-walled polypes there is no 

 invagination of the tentacles — they are simply withdrawn ; but in those whose polypes 

 are thin-walled a partial invagination takes place. Even if there were any value in 

 this character it is not one, I think, that should be very readily adopted, as it is not 

 very easy to determine the mode of retraction of the tentacles with certainty without 

 making a series of sections through a large number of polypes, and it would consequently 

 be a character of great inconvenience to the systematic zoologist. Until some other 

 character has been described, the genus Bhizoxenia must go, and consequently 



RMzoxenia rosea, Dana (3), becomes Clavularia danm^ 

 Rhizoxenia primula, Dana (3)^ becomes Clavularia primula. 

 RMzoxenia thalassantha, Ehrenberg (5), becomes Clavularia thalassantlia. 

 Rhizoxenia filiformis, Sars (26), becomes Clavularia filiformis. 



The genus Sarcodidyon was established by Forbes (12) for a delicate little form with 

 very thin-walled polypes, which may be completely withdrawn into a thin ramifying 

 stolon of ribbon-like bands. 



There can be no doubt that the Bhizoxenia filiformis of Sars is closely related to 

 Sarcodidyon catenata of Forbes ; but it was hardly fair of Sars to criticize Forbes's 

 action in not including it in the genus Bhizoxenia. Surely it was Sars who was at 

 fault in placing his species with perfectly retractile polypes in a genus whose main 

 character was that the polypes were not retractile ! 



However, I agree with Sars that Sarcodidyon must be given up, and consequently 

 Sarcodidyon catenata of Forbes becomes Clavularia catenata. 



It is difficult, too, to find any very definite character by which to distinguish the 

 genus Anthelia from Clavularia. The stolon is a membranous expansion, the polypes 

 are not retractile as a whole, though their tentacles are, and there are numerous 

 spicules. All of these characters are to be seen in some Clavularias. 



Dana says, " The Anthelice cover the rocks or any solid support at hand with thin 

 fleshy plates, which consist of an aggregation of polypes united by their bases. They 

 dififer from the Xetiiw in budding only at the base, which gradually spreads outward by 

 the process, producing finally the encrusting plate." 



'^ The name Clavularia rosea has been given to a sjjecies from Kergiielen by Studer (28). 



