PART I.— ON THE HYDROCORALLIN^. 



INTRODUCTION. 



This Part of the Report contains an account of the structure of Millepora nodosa, and 

 a Monograph on the structure of the Stylasterid^e, together with a list of the species 

 dredged by H.M.S. Challenger, and a further list of all species hitherto described. It 

 is divided into three Sections, as follows : — 



Section I. — On the Structure of the Milleporid^e. 

 Section II. — On the Structure of the Stylasterid^e. 

 Section III. — General Remarks on the Hydrocorallin^e. 



It is mainly a reproduction of two papers on the structure of Hydroid Corals which were, 

 by the permission of the Lords Commissioners of the Treasury, published as parts of a 

 Preliminary Report on the results of the Challenger Expedition in the Philosophical 

 Transactions of the Royal Society for 1877 and 1878. 1 Certain alterations have, how- 

 ever, been made in the account of the structure of Millepora, which were necessitated 

 by the discovery, subsequent to the appearance of that paper, of the Hydroid nature of 

 the Stylasteriixe. The terms made use of have been altered where necessary in order to 

 render them uniform throughout, and the whole has been revised and to a considerable 

 extent rearranged. The list of Stylasterid.e dredged by H.M.S. Challenger, with descrip- 

 tions of a few new species, is a new addition. The zoological relations of the Milleporid^e 

 and of the Stylasterid^e are discussed separately at the ends of the parts of the treatise 

 relating to each of them, and a general discussion on the relations of the Hydrocorallin^e 

 as a whole, and on their pedigree, concludes the Monograph. 



1 On the Structure of a Species of Millepora occurring at Tahiti, Society Islands (Philosophical Transactions of 

 the Royal Society, 1877, p. 117) ; The Croonian Lecture, On the Structure of the Stylasterid^e, a family of Hydroid 

 Stony Corals (ibid., 1878, p. 425). 



