INTRODUCTION. lix 



soon lay the foundations of a new Hydroid colony, was 

 relegated in the systems of classification to a distance 

 from all its nearest of kin. 



This primary and inevitable mistake has introduced a 

 large amount of confusion into this department of zoology, 

 and we are only now escaping, in part at least, from the 

 effects of it. 



The most important result of recent investigation has 

 been the union of the Hydroid zoophytes and the naked- 

 eyed Medusae of authors in one great natural group. The 

 two forms of structure embraced in this division, the one 

 represented by the Hydra, and the other by the (so-called) 

 Jelly-fish, which appeared so dissimilar when only known 

 in isolation, are now proved to be essentially identical : 

 the fixed and floating polypites are but different phases of 

 one and the same organism. And these elements are 

 variously manifested and combined in the Hydroid 

 group. In some cases there are fixed zooids (alimentary 

 polypites) and free zooids (sexual polypites) developed from 

 the same stock, and constituting one (zoological) individu- 

 ality. In other cases there are two classes of fixed zooids, 

 the nutritive and reproductive, permanently united ; in 

 others, again, there are only free zooids (floating poly- 

 pites) in which the nutritive and sexual functions are 

 combined. But these are in reality nothing more than 

 variations of one and the same structural group. 



There has been considerable diversity of opinion as to 

 the true position of the small number of medusan * forms 

 that are developed directly from the ovum without the 

 intervention of any fixed Hydroid stock. But they certainly 



* This term is employed here and elsewhere as an adjective, descriptive 

 of a certain modification of Hydroid structure. 



