viii PREFACE. 



in their living state be thus conveyed, but greater facilities will be afforded to the 

 practical zoologist in the comparison and determination of species. The plates, 

 moreover, contain numerous anatomical and embryological details ; and, besides the 

 magnified drawings of each species, I have in every case given a figure of the 

 animal in its natural size. 



It was originally my intention to restrict the descriptive portion of the work 

 to the British representatives of the group. Further consideration, however, has led 

 me to believe that its value would be much increased by including descriptions of all 

 the known Gi/mnoUastea, whether British or foreign. The plates, however, are 

 necessarily confined to British species. Indeed, independently of other reasons, this 

 course was inevitable so long as I had resolved to make all my drawings from living 

 specimens. Full reference, however, is always given to the places where published 

 figures of the foreign species are to be found. 



The same reason has obliged me to leave a few British species unfigured, as 

 I have hitherto failed in my attempts to obtain living specimens of them. Eeferences, 

 however, are here, as in the case of foreign species, always made to the works in 

 which figures of them are given. 



Besides the plates, numerous woodcuts are introduced into the text. Though a 

 few of these have already appeared in my published memoirs, they are all from original 

 drawings of my own, and will, it is hoped, serve to render clear various points of 

 structure which it would be difficult to make intelhgible without the aid of figures.' 



For obvious reasons it is only those species whose trophosomes have been dis- 

 covered which form the subject of the descriptive portion of the present work. 

 There are still known to zoologists a large number of hydroid medusae which have 

 not yet been traced to their trophosomes. Since Forbes's Monograph, published 

 among the earlier volumes of the Bay Society, much additional matter has been 

 accumulated regarding these beautiful organisms, and many of them have been 

 figured with structural details in the first part of the present work. I have still 

 many unpublished notes on them, and, though it was impossible to treat them here 

 systematically, I cannot dismiss the hope of being yet able to supplement the present 

 volume by another which would be devoted to the natural history of these free 

 hydroid medusae, whether they have been traced to their trophosomes or not. 



As the descriptive portion of this Monograph is based upon the entire organism, 

 both trophosome and gonosome affording characters equally essential in the diagnosis, 

 I have never been contented with specimens in which the gonosome as well as the 

 trophosome was not present. It is only in one or two cases that I have failed in 



^ The use of the blocks employed in the illustiation of my " Report on the Hydroida," pub- 

 lished in the ' Transactions of the British Association for the Advancement of Science/ has been 

 liberally granted by the Council of that body. 



