INTRODUCTION. XXXV 



polypite in this zoophyte is somewhat concealed by the 

 peculiar habit of the former, as it is seen striding along 

 with inverted mouth. But in the kindred genus Eleutheria 

 the special locomotive organ is wanting, the mouth of the 

 gonozooid is turned upward as it moves, and it has all the 

 appearance of a polypite propelling itself, with little appa- 

 rent ease or agility, by means of its tentacles. 



So much may suffice respecting the structural identity 

 between the two principal elements of the Hydroid colony 

 the polypite and the (so-called) medusa, which, on a 

 superficial view, appear to offer such a complete and 

 striking contrast *. 



In the free sexual zooid with its contractile bell and 

 mercurial habit, which not only matures but also diffuses 

 the seed of new generations, the hydroid structure reaches, 

 as it were, its culminating point. 



In a large proportion of cases, however, the reproductive 

 element appears in much humbler guise. The gonozooid 

 is permanently attached to the colony, like the alimentary 

 polypite, and developes and liberates its products in situ. 

 Amongst these fixed zooids, which are extremely numerous, 

 a gradation of structure is traceable. A series of transi- 

 tional forms connects the simplest of them, which is a 

 mere sac, with the most complex, which makes a near 

 approach to the medusa in structure, though not destined 

 to become free. In different species the development of 

 the gonozooid is, as it were, arrested at different points ; 

 and it is only in certain cases that it attains the highest 



* I have not attempted to give an exhaustive view of this deeply interesting 

 portion of my subject. Those who desire a fuller treatment of it may con- 

 sult the works of Prof. Allman, and especially his admirable paper on C'or- 

 di/lophora (Phil. Trans. June 1853), and his " Eeport on the reproductive 

 system in the Hydroida." Report Brit, Assoc. for 18<>3, pp. 3 



c2 



