PREFACE. ix 



procuring examples provided with their gonosomes, and that I have been obliged 

 to confine my figures to the trophosome alone. 



The additions which the last few years have made to our knowledge of hydroid 

 morphology liave necessitated the introduction of new terms. Such terms as I have 

 found it necessary to construct have been made as far as possible etpuologically 

 significant of the ideas intended to be expressed by them, while I have endeavoured 

 to define them with a rigidity which may allow of no ambiguity in their application. 

 The advantages to be derived from a significant and rigidly defined terminology 

 are great, for it not only facilitates the recording and communication of scientific 

 truths, but it even becomes, like the symbols in algebra, a direct aid in original research. 



With the view of making the terminology as perfect as possible, I have not 

 hesitated to alter some of the terms formerly introduced by myself. Terminology 

 differs from nomenclature in priority of use not necessarily giving a fixity of tenure ; 

 and while capricious change of terms must be deprecated, no one ought to be 

 precluded from substituting a better term for one already in use. 



The labour of the drawings, which I could entrust to no hand but my own, and 

 the necessity of procuring in every case living specimens as the subjects of them, 

 have caused the work to be longer in preparation than I had originally anticipated, 

 and I cannot avoid here expressing my obligations to the Council of the Ray Society 

 for the patience with which they have borne the delay. One advantage, however, has 

 followed from it, for I have been thereby enabled to carry up to the present stand- 

 point of our knowledge this exposition of a rapidly developing department of research, 

 in which every year has been bringing out new facts and more or less modifying old 

 views." 



The coasts of the British Isles have afforded me the chief fields for exploration, 

 and my dredgings and tidal coast work have extended from the south-western extre- 

 mity of Cornwall to the furthest outliers of the Shetland Isles. Some investigations, 

 however, have been also carried on in the Mediterranean, and I have thus obtained 

 many facts in hydroid zoology from the northern shores of the Adriatic, from the coast 

 of Naples, and from the eastern and western Riviera. 



Continental museums, wherever accessible, have been consulted. These, on the 

 whole, are very poor in all that concerns the zoology of the Htdroida, and few of them 

 possess anything beyond some dried specimens of such common species as may be 

 casually picked up on the sea-beach. 



Some, however, have repaid the trouble of consultation, and I must here express 

 my thanks to M. Milne-Edwards and to M. Lacase Duthiers for the liberal manner in 



' Quite recent additions to our knowledge of hydroid life render necessary some modification of 

 the statements contained in pp. 22, 23 regarding our want of evidence of the direct development of the 

 medusa from the egg, without the intervention of a hydriform trophosome. The reader will accordingly 

 correct and supplement these statements by the results of later observations detailed in p. 100. 



b 



