10 THE IIYDROIDA IN GENERAL. 



Almost precisely at the same time appeared the first edition of the ' Regne Animal' of 

 Cuvier.^ The general division of the animal kingdom, of which, as we have already seen, a 

 sketch had previously appeared, is here adopted and carried into detail. In his association of 

 the free Hydrozoa into a distinct class under the name of Acalepha, Cuvier now takes an 

 important step, though its value is deteriorated by the admission of Actinia into the same 

 group. Independently of the advance which the natural history of the Hydroida has thus 

 directly received, the great influence which Cuvier has exerted on the studies of the zoologist, by 

 taking anatomical structure rather than external resemblance as the basis of classification, renders 

 it impossible in the history of any department of zoology not to see in the puljlication of the 

 ' Regne Animal' a well-marked era of development. 



The voyage of Perou and Lesueur, which had such valuable results for zoology, was only the 

 first of a long series of scientific expeditions which, fitted out under the auspices of various 

 governments, brought back with them rich stores of materials, and mark out the first half of the 

 present century as eminently the era of the naturalist voyager. 



Between the years 181.5 and 18:26 two exploratory expeditions round the world were fitted 

 out by tlie Russian Government." They were entrusted to the command of Kotzebue, and were 

 accompanied by Chamisso and Eschscholtz as naturalists. The expeditions afforded fine oppor- 

 tunities for the observation of pelagic forms of Mo/lttsca and Coelenterata, and are rendered 

 memorable by Chamisso's famous discovery of the " alternation of generations " in Saljui — a 

 discovery which was destined to exert great influence on the study of the Hvdroida and the 

 interpretation of their marvellous life-history. 



The study of the ccelenterate animals observed during these voyages was specially under- 

 taken by Eschscholtz, and after the return of the voyagers from their second expedition we find 

 this philosophic naturalist publishing a general work on the Medusie and allied forms ' — a work 

 by far the most important which had as yet appeared upon the animals of which it treats, and 

 one which, even at the present day, the student is unable to dispense with. 



It is here that, for the first time, we find the hydroid Medusae, under the name of " Discophora 

 cryptocarpa," separated as a distinct and well-defined group from the proper Discophora, to 

 which Eschscholtz assigns the name of " Discophora phanerocarpa ;" and though the characters 

 on which this dismemberment was based were but imperfectly understood by Eschscholtz, and 

 have since undergone considerable modifications, the conception of the hydroid Medusas as a 

 separate section is a step of primary importance, and could have been entertained only by one 

 who was able to recognise the fundamental differences and appreciate the true affinities of the 

 group among which these organisms had been hitherto indefinitely distributed. 



Cuvier had already * recognised an essential difference of structure between the actinozoal 

 and hydrozoal forms included in his group of " Polypes," when he pointed out the presence of a 

 digestive sac, with differentiated walls in the former, and its absence in the latter ; but the 



^ Geo. Leop. Chr. Fred. Dajob. Cuvier, ' Le Regno Animal distiibue d'apres son organisation, pour 

 servirde base il I'Histoire Naturelle des Animaux at d'introduction a I'Anatomie comparee,' Paris, 1817. 



" Otto V. Kotzebue, ' Voyage of Discovery into the South Sea and Behring's Straits, undertaken 

 in 1815-18, in the ship " Rurick," ' Loudon, 1821 ; and ' New Voyage round the World in 1823-26,' 

 London, 1830. 



' Job. Priedr. Eschscholtz, ' System der Acalephen,' Berlin, 1829. 



* 'Le Regne Animal,' 1817, tome iv, p. 79. 



