190 Royal Society: — 



Conclusions. 



Since the observations of Prof. Sars* on the polyps of Allopora 

 oculina it has been to some extent suspected that the Stylas- 

 teridae were not Anthozoa, but possibly allied to the Milleporidae, 

 although the fact was not in any way demonstrated. Milne-Ed- 

 wards long ago expressed himself extremely uncertain as to the 

 affinities of Distichopora, and suspected that it might be an Alcyo- 

 narianf. In consideration of the facts now ascertained, there 

 can be no doubt as to the hydroid affinities of the family. The 

 Stylasteridse appear to form a very natural family. They all 

 possess two kinds of zooids. The tentacular zooids are closely 

 similar in form in all the genera ; and in the variations in the 

 forms of the alimentary zooids all gradations are present. The 

 thread-cells appear to be alike in form in all the genera. In all 

 the gonophores are developed within ampullae. The corals all bear, 

 as far as has yet been ascertained, fixed sporosacs, as do, according 

 to Allman, all deep-sea Hydroids J. It is possible, however, that 

 forms such as Stylaster sanguineus occurring in shallow water § 

 may bear planoblasts. There can be no doubt that Distichojwra 

 will prove closely allied to the other six genera of Stylasteridse : 

 its well-marked ampullae and two kinds of pores are decisive in 

 the matter. Pliohothrus is said by Pourtales i| to have ' occasional 

 round cavities in the centre of its branches filled with a yolk- 

 like substance contained in a membrane." These cavities seem to 

 be ampullae : and if so, then Pliohothrus may prove to belong to 

 the Stylasteridse, and not to the Milleporidae. In a specimen of 

 Pliohothrus obtained by the ' Challenger ' I have been able to detect 

 neither ampullae nor tabulae. It will evidently be possible easily 

 to form natural genera for the Stylasteridae characterized by the 

 number of tentacles of the alimentary zooids, grouping of the tenta- 

 cular zooids around them, &c. This I propose to attempt when I 

 have completed my study of the subject. 



The Milleporidae differ from the Stylasteridae in having tabulae, 

 and in possessing neither styles nor ampullae, as well as in having 

 their mouthless zooids provided with numerous tentacles. The 

 two families have, however, many points of alliance, and they 

 should, provisionally at least, be referred to a special suborder of 

 the Hydroidea, which may be termed the Hydrocorallinae. 



A most remarkable result of the present inquiry is the de- 

 termination that the calicles of Stylaster and Cryptohelia are 

 tenanted and formed by colonies of zooids, and not by single 

 polyps, as was most naturally hitherto supposed to be the case. 

 Prof. Verrill, in criticising Prof. Agassiz's relegation of the Eugosa 



* Sars, Forh. Selsk. Christ. 1872, p. 115. 



t MM. Milne-Edwards and Haime, I. c. t. iii., Appendice, p. 451. 



X Allman, I. c. vol. ii. p. 155 ; also ' Nature,' Oct. 28th, 1875, p. 666. 



§ Pourtales, I. c. p. 83. 



11 Pourtales, I. c. p. 67. 



