HISTORY OK RESEARCH. 3 



zooiils of the TubulariiHc and CaMpaiiulariiuc, wliile those which have not yet been so traced — 

 provided we have no reason to regard them as the free zooids of the S'iphonophora — and even 

 those which may be proved to be developed directly from the egg, cannot, in a philosophical 

 system, be separated from the others. 



1 accept without hesitation the group CcELENTFaiAT.\, witii the characters assigned to it by 

 Leuckart; and I further adopt the division of this group into two primary sections, with the 

 names of Adbwzoa and Hydrozoa, as projjosed by Huxley. The following table will indicate 

 the place of the Hydroida among the other members of the llijdrozoa .' 



ACTINOZOA. 



CCELENTERATA 



Cteuopliora." 

 Discopliora. 

 Hydhozoa . . / LuceniariBe. 



Hydroida. 

 y Siplionophora. 



HISTORY OF THE PROGRESS OF OUR KNOWLEDGE OF THE HYDROIDA. 



The history of the successive stages through which any important branch of human 

 knowledge passes in its development from the first dawnings of its truths upon the mind to that 

 more perfect phase which in the lapse of time it has attained, constitutes one of the most instruc- 

 tive subjects upon which the philosophic student can be engaged ; and a history of this develop- 

 ment, as it shows itself in the j)i-ogress of our knowledge of the Hydroida may, therefore, with 

 advantage precede that exposition of the present state of oiu- knowledge of them which is the 

 chief aim of the present work. 



In order to avoid extending our historical sketch to an inconvenient length, the record of many 

 unportant anatomical and physiological discoveries must be postponed to that part of the volume 

 where these discoveries can be described with sufficient detail ; and I shall here confine myself 

 chiefly to the more important steps which have been made towards the determination of the 

 systematic position of the Hydroida, and their recognition as a distinct group with the limits 

 assigned to them in this monograph. 



' I must for the present hesitate to include among the liydrozoal orders the tabulate and rugose 

 corals. The hydrozoal affinities of these groups have been recently claimed for them by Agassiz as the 

 result of an examination of living specimens of MiUepora alcicornis, a tabulate coral, in which, if there 

 be no error of observation, Agassiz has detected a true hydrozoal structure, while he believes himself 

 supported by analogy in attributing this structure, not only to all the other genera of tabulate corals, 

 whether living or extinct, but even to the entirely extinct group of Rugosa. (See his ' Cont. Nat. 

 Hist. United States,' vol. iv.) The observations, however, on which this view has been based are 

 plainly not yet as complete as could be desired for a determination so important, and even startling. 

 Of the generative system more especially we are entirely ignorant. Under these circumstances I 

 believe it will be safer to wait for such verification as may be expected from further researches. 



' In adopting the more usual view, in accordance with which the Ctenophora are placed among 

 the Hydrozoa rather than among the Actinozoa, as originally, indicated by Leuckart, and more deci- 

 dedly insisted on by Huxley, I believe myself borne out by a careful study of the structure of Beroe. 



