18 HISTORY OF RESEARCH. 



tliis work a spccinl value, and place it among the most important contributions we possess to the 

 natural history of the IItdroida. 



Agassiz here divides his " Acalephaj" into three orders, CtenopJiora, Biscophora, and 

 llydroida, which last embraces the Gijmnnpldhahnia of Eorbes, the admitted polypoid forms of 

 Hydrozoa, and the Sijjhonopkora and Lucernarim ; and he also, contrary to the universally enter- 

 tained opinion of previous naturalists, maintains that the corals constituting the still living group 

 of the Tahulata, and those forming the entirely extinct groups of the Tubulosa and Uugosa, have 

 a true hydroid structure, and that they must accordingly be removed from the Adinozoa, with 

 which they had been hitherto associated, and take their place among the genuine Hydroida. 



It will be seen that Agassiz here follows Kolliker, Leuckart, and M'Crady in uniting the 

 polypoid forms of Hydrozoa with the gymnophthalmic Medusae as elements in a group equivalent 

 in value with that of the Biscophora, a name which he employs in the limited sense adopted by 

 Kolliker to indicate the Biscophora phanerocarpa of Eschscholtz. 



In 1861 Greene published his excellent little Manual of the Coelenterata,^ a work which gives 

 us in a condensed form a very complete view of the structure, development, and relations of the 

 various members of this group. He adopts the name of Hydrozoa in the sense in which it 

 was limited by Huxley, while he combines the gymnophthalmic Medusa? into a distinct order 

 from which the polypoid forms are excluded, and thereby as, I believe, fails to express the true 

 relations of these organisms. 



Though Greene's Manual lays no claim to originality, it discusses in a philosophic 

 spirit various questions bearing on the subject with which it is occupied, and constitutes one of 

 the most valuable aids to the general study of the Cwlenterata which can be placed in the 

 hands of the student. 



In the first volume of the ' Manual of Zoology,' by Peters, Carus, and Gerstaecker," pub- 

 lished in 1863, J. Victor Carus gives us among other articles an excellent one on the Cwlenterata. 

 The Hydromediism which form his third order of Ccelenferata are here divided into the Siphono- 

 phora and Hydroidea, the latter embracing both the free gymnophthalmic Medusae and the 

 polypoid colonies. The article contains an account of the leading anatomical and embryolo- 

 gical features of the Hydroida ; and the subordinate groups under which the author believes 

 that they ought to be distributed are characterised. It also contains a very useful synopsis with 

 diagnoses of all the genera of hydroids. 



Observations on both the hydriform and medusiform elements of the Hydroida had been 

 thus for several years accumulating, and the time had already come when it seemed possible to 

 assign to the free hydroid Medusa its proper place in a comprehensive system of the Hydroida. 

 The importance of uniting the two elements in the definitions of genera had been already recog- 

 nised by M'Crady, Agassiz, and Victor Carus ; but notwithstanding the prominence which these 

 authors, and especially Agassiz, had given to the medusiform buds, it did not seem that a 

 thoroughly natural distribution of the Hydroida under legitimately limited genera had yet been 

 effected, and this belief led me in 1864 to attempt a revision of the older genera of all tubularian 

 and campanularian hydroids whose hydriform element was known.' 



' Joseph Ileay Greene, 'A Manual of the Sub-kingdom CcBlenterata,' London, 1861. 



^ Wilh. C. H. Peters, Jul. Victor Carus, und C. E. Adolph Gerstaecker, ' Handbuch der Zoologie,' 

 Leipzig, 1863. 



' Allman, ' On the Construction and Limitation of Genera among the Hydroida.' ' Ann. and 

 :\rag. of Nat. Hist.' for May, 1864. 



