HISTORY OF RESEARCH. 15 



by which the two divisions may be anatomically characterised. In 181S I'orbes's views were 

 further developed in a beautiful monograph on the Naked-eyed Medusae of the British seas,' 

 in which every known British species of hydroid Medusa is described and illustrated by 

 an original figure. In this work he distinguishes the two groups by the names of " Stcganoph- 

 thalmia," corresponding with the Discophora plianerocarpa of Eschscholtz, and " Gymnoph- 

 thahnia," corresponding with his Discophora cryptocarpa. AVitli Forbes's monograph we may 

 date the definite acceptance of the hydroid jMedusae as a well-marked and legitimate group. 



The Radiata of Cuvier, which, by the elimination of various groups originally included in it, 

 had been gradually attaining to a form more in accordance with the requirements of a natural 

 classification, was in 1847 subjected to an important revision by Leuckart," who insisted on the 

 necessity of attending to a remarkable type of structure, which was common to certain members 

 of the Radiata as then accepted by zoologists, namely, the free communication of the general 

 body-cavity with the external world through the mouth. He saw in this feature a character of 

 great value by which, after the exclusion of the Polyzoa, the whole of the Cuvierian " polypi " 

 and " acalepha " would requii'c to be united into a distinct group apart from the EcJiinodermata. 

 To the group thus constituted Leuckart gave the name of Ccdenterata ; while soon after Huxley 

 was led to adopt similar views, and, quite independently of Leuckart, proposed the construction 

 of an exactly equivalent group, under the name of Nematophora, suggested by the universal 

 presence of thread-cells in the tissues of the animals composing it.^ 



The relation between the fixed hydroid trophosomes and the free hydroid meduste, which, 

 as we have seen, had already become apparent, received about this time additional light 

 from the researches of Dalyell,* an acute, laborious, and conscientious observer, though without 

 that technical precision in his descriptions which indicates a special zoological training, and 

 without much acquaintance with what had been already done by others. Dalyell has recorded 

 many additional instances of the development of Medusae from the hydroid trophosome ; and the 

 accumulation of such facts now began to exert an influence on the classification. 



We accordingly find Carl Vogt in 1851 combining the fixed hydroids with the whole of the 

 Discophora of Eschscholtz, in order to form a single group, to which he gives the name of 

 Hydromedma!' 



In 1853 we find Kolliker employing the name of Hydromedusida^ but restricting it to a 

 group composed of the gymnophthalmic Medusse of Forbes, the Siphonophorous genus Velella, 

 and aU the non-natatory hydi-oids, except Hydra, which he unites with the rest of the Siphono- 

 phora to form his group Hydroiden, while he confines the name Discophora to the Sleyanoph- 

 thalmia of Forbes, or Discophora phanerocarpa of Eschscholtz. 



These proper Hydrozoal groups are, along with the Ctenophora, united to the Actinozoa, in 

 order to form his division Radiata molluscoida, which would thus constitute a great natural 



' Edward Forbes, ' A Monograph of the British Naked-eyed Medusse,' London, 1848. Published 

 by the Ray Society. 



• Hnr. Frey und Rud. Leuckart, ' Beitrage zur Kenntniss wirbelloser Thiere,' Braunschweig, 184". 

 ' Tb. H. Huxley, " An Account of Researches into the Anatomy of the Hydrostatic Acalepha;," 



' Brit. Assoc. Rep.' for 1851. 



* Sir John Graham Dalyell, ' Rare and Remarkable Animals of Scotland,' Loudon, 1847. 

 " Carl Vogt, 'Zoologiscbe Briefe,' Frankfurt-a-M., 1851. 



^ Albert Kolliker, ' Die Schwimmpolypen oder Siphonophoren von Messina,' Leipsig, 1853. 



