Mil INTRODUCTION. 



in such matters may consult Johnston's ( History/ where 

 they will find a careful review of all that had been done 

 in this department from the time of Ellis downwards and 

 the third volume of Agassiz's ( Contributions to the Natu- 

 ral History of the United States/ 



Until a very recent period the real facts of the Hydroid 

 life-history had not been fully ascertained, and the basis 

 of a natural arrangement was therefore wanting. Even 

 when Johnston wrote the true nature of the medusiform 

 zooid had not been determined, and he followed Van 

 Beneden in regarding it as the embryo. At that time, 

 also, a very small number of the (so-called) rnedusoids had 

 been traced to their Hydroid stock, and the naked-eyed 

 Medusae were still treated as a group distinct from the 

 Hydroida. The accumulation of facts has proceeded 

 steadily since that period ; but the correct interpretation of 

 them and the elaboration of a really philosophical classi- 

 fication are amongst the latest results of research. 



The Hydroid community presents two dissimilar ele- 

 ments, discharging respectively the functions of alimen- 

 tation and reproduction ; and in a large number of cases 

 these two elements separate from one another at a cer- 

 tain stage, and lead thenceforth an independent existence. 

 Before the connexion between these sundered parts was 

 recognized, and while they were only known as distinct 

 and dissimilar organisms, they were ranged under different 

 classes and distinguished by different names. A double 

 nomenclature was invented to designate what were only 

 fragments of one and the same individuality. Integral 

 portions of the same being were treated as if there were 

 no affinity between them ; and the zooid which had but 

 lately detached itself from the Hydroid stock, and would 



