HISTORY OF RESEARCU. 19 



In the diagnoses of tlie genera I regarded the reproductive zooid, wlietlier fixed or free, of as 

 much importance as the nutritive, and the resulting classification, in so far as it applies to the 

 subject of the present Monograph, is, with such modifications as further investigations rendered 

 necessary, that which I have here adopted. 



At this period the yUffinidis constituted a group of Medusae whose structure was but imper- 

 fectly understood, and wdiose systematic position and affinities had given rise to nuich discussion. 

 In 1865, however, Ernst Haeckel published some very valuable observations^ which no longer 

 leave any doubt that these Medusae possess a true hydroid structure, by which tiiey become asso- 

 ciated with the ordinary hydroid or gymnophthnluiic Medusje ; while he still further made the 

 remarkable discovery that Citnina, a typical ^Eginidan, is produced as a bud from Geryonia, a 

 medusa of an entirely different form, and one whose true hydroid affinities had never been doubted. 



In the same year, Alexander Agassiz publislied his illustrated Catalogue of the North 

 American Hydrozoa contained in the Museum of Comparative Zoology at Harvard College.'^ 

 In this work, besides other hydrozoal groups, a large number of hydroid Medusae, occasion- 

 ally accompanied by their trophosomes, and including many new forms, are described and illus- 

 trated by very expressive woodcuts. The views of Prof. Louis Agassiz on almost all that 

 concerns nomenclature, generic groups, and systematic position and affinities are throughout 

 adopted by his son ; but whether these views be in all points accepted or not, we cannot but 

 regard the work of Alexander Agassiz as tending in no small degree to advance to our knowledge 

 of the zoology of the Hydkoida. 



In 1S6G, while the present sheets were passing through the jiress, M. Van Benedcn 

 published in the ' Memoirs of the Royal Academy of Belgium ' a treatise on the Natural 

 History of the Ccelciilerata of the Belgian Coast, a group for which he uses the general name of 

 " polypes."' 



The greater part of the work is devoted to the Hydroida ; it is illustrated by many 

 beautiful plates, and is one of the most elaborate treatises which has been hitherto published 

 on these animals. M. Van Beneden describes and figures not only a considerable number of the 

 fixed forms, but also many free medusiform gonophores ; and the work must be regarded as a 

 valuable contribution to the iconography and descriptive natural history of the Hydroida, though 

 the author's opinions on many points, more especially such as concern the synonomy and 

 determination of species, cannot receive our assent, and will be discussed in another part of the 

 present ^lonograph. 



While the natural history of the Hydroida was thus gradually advancing towards perfec- 

 tion in all that appertains to descriptive Zoology and systematic arrangement, the anatomists were 

 developing it from a structural and embryological point of view. The introduction of the achro- 



^ Ernst Haeckel, ' Ueber eiue iieue Form des Generationswechsels bei den Medusen, und iiber 

 dieVerwandeschaft der Geryonideu und Aegmiden Monatsbericht der Kouige Akad. der Wiss. zu Berlin/ 

 2 Feb., 1863. Translated in the 'Ann. of Nat. Hist.' for Jan. 1865. 



Id., ' Beitrage zur Naturgescliiclite der Hydrottiedusen,' Leipzig, 1805. 



■ ' Illustrated Catalogue of the Museum of Comparative Zoology at Harvard College.' No. II. 

 "North American Acalephee." By Alex. Agassiz. Cambridge, U.S., 1805. 



' P. J. Van Beneden, " Recherclies sur la Faune littorale do Belgique. — Polypes." ' Mem. dc 

 I'Acad. Roy. de Belg.,' tome xxxvi. Published also in a separate form. 



