2 GENERAL INTRODUCTION. 



and my suspicions that it belonged to the Hydroids were confirmed by the examination 

 of the remarkably rich haul of Stylasterid.e obtained on the homeward voyage off the 

 mouth of the Rio de la Plata. 



In the Hydroid corals of Hydrocorallin.e the hard skeleton appears to be developed 

 from the ectoderm, whereas in the Anthozoan corals, both Alcyonarian and Madreporarian, 

 it is not so produced. It seems necessary, therefore, that the term "corallum" should not 

 be applied to the hard skeletons of both forms alike, since the skeletons in the Hydroid 

 and Madreporarian corals can scarcely be considered homologous. I have, therefore, 

 applied the term " ccenosteum " to the hard skeleton of the HydrocoralUnae, retaining 

 the old term " corallum " for that of the Anthozoan corals, whether Alcyonarian or 

 Madreporarian. 



The present memoir is divided into three parts. The first part treats of the 

 Hydrocorallin.e, giving an account of the structure of the Milleporid.^ and 

 Stylasterid,e, and a list of all known species of Stylasterid^e, together with descrip- 

 tions of the species obtained by H.M.S. Challenger. The second part describes the 

 anatomy of Heliojwra ccerulea and of a species of Sarcophyton , and contains a discussion 

 on the fossil corals which were probably allied to Heliopora. The third part deals with 

 the Madreporaria dredged in the deep sea. It comprises a list of all the species 

 dredged, with descriptions of new forms, and some account of the anatomy of the soft 

 tissues of certain species, and a table showing the depths to which all known genera of 

 deep-sea corals are as yet known to range. The literature relating to the matter treated 

 of is shortly discussed in each part separately. 



