16 THE HYDROIDA IN GENERAL. 



group correspoiuling to the Ccelenterata of Leuckart, were it not that KoUiker unfortunately 

 combines with the coelenterate forms the Poli/zoa, so as to form by the union of the two his 

 Radiata molluscoidea. 



The same year Leuckart' jiroposed to divide the whole of the Ccelenterata into three classes 

 — the Ctenophora of Eschscholtz, composed of the Beroes and their allies ; the " Medusae " 

 (Scheibenquallen), and the " Polypes," which last name he confines to the Actinozoal Cwlenterata. 

 Under the " Medusse " of this classification are comprised three orders — the Dkcophora proper, 

 tiie llydroida with their Mediisffi, and the SipJionophora. 



The union thus nearly simultaneously proposed by Kolliker and by Leuckart of the non- 

 natatory forms of hydroids with the gymnophthalmic Medusae so as to form a group distinct from 

 tluit of the steganophthalmic Medusae, is the first expression we have of a natural relation which 

 ail subsequent research has oidy tended to confirm. 



\n 18.56 Leuckart published his Supplement to Van der Hoven's 'Manual of Zoology. ^ In 

 this valuable little work we find him distributing the gymophthalmic Medusae between two 

 groups. Those which he believes to undergo a direct metamorphosis without the intervention of 

 a hydroid trophosome are combined into one order, to which he gives the name of Ceratostera, 

 from the rigid habit of their marginal tentacles ; while those which are known to be produced as 

 buds from a trophosome are associated with their trophosomes, and with other fixed hydroids in which 

 the gonophore never assumes the form of a free medusa, so as to constitute his order Hydroidea. 



In the same year Huxley' proposed to divide the group Cwlenterata into two classes, which 

 he names Hydrozoa and Acfinozoa. Instead of assigning to the Ctenophora the value of a 

 distinct class, he regards them as Actinozoa, thus bringing them into immediate relation with 

 Actinia. 



In 1857 Gegenbaur published a very valuable paper on the Medusae,* in which he describes 

 and figures many new forms of Gymnophthalmia, the subordinate groups of which he revises and 

 limits more in accordance with the natural affinities of the animals than had been hitherto 

 attempted. He adopts the Eschscholtzian subdivision ; but instead of basing it on the characters 

 assumed by Eschscholtz, or on those proposed by Forbes, he finds the grounds of the subdivision 

 in the presence or absence of the membranous diaphragm which extends horizontally inwards 

 from the margin of the unibclla, and which Forbes had already designated by the name of 

 "Velum." In accordance with this view Gegenbaur divides the Medusae into the Acraspeda, or 

 those in wliicli no velum is developed, and which correspond to the Steganoplithalmia of Forbes, 

 and the Craspedota, or those which are provided with a velum, and which correspond to the 

 Gymnophthalmia of Forbes. 



In 1858 McCrady published an important paper on the " Gymnophthalmic Medusae,"^ in which 

 many new forms are described and figured. Instead, however, of limiting his group Gymnoph- 

 thalmia within the bounds of that to which Forbes had already assigned this name, he extends 



' Rud. Leuckart, ' Zoologiscbe Untersuchungen.' Erstes Heft. Giessen, 1853. 



^ Eud. Leuckart, ' Naclitrage nud Berichtiguugen zu dcm ersten Bande von J. Van der Hoven's 

 Handbuch der Zoologie,' Leipsig, 1856. 



^ Thomas Henry Huxley, " Lectures on General Natural History," in ' Medical Times ' for 1856. 



* Carl Gegenbaur, " Versuch eiues Systemes der Medusen," 'Zeitsch. fiir Wisseusch Zoologie,' 1857, 



'•" J. McCrady, the Gymnophttialmata of Charleston Harbour, ' Proc. of the Elliott Society of 

 Natural History,' Charleston, 1858. 



