64 



IXVEIITEBEATE AXIMALS. 



bell becomes partially closed by an inward prolongation or shelf, 

 called the "veil"; and a, series of tentacles are developed from its 

 margin. The generative bud, thus liberated, leads a wholly inde- 

 pendent existence. The manubrium, having developed a mouth, 

 assumes the functions of a true polypite, and its cavity acts as a 

 digestive sac. The whole organism swims about freely, and has the 

 power of assimilating food, and thus of attaining to a comparatively 

 gigantic size. This independent existence, however, only goes on 

 till such time as the elements of reproduction can be produced. 

 The ova and sperm-cells are developed in specialised portions of this 

 generative bud, and then it ceases to exist. The ova, however, 

 when fertilised, do not develop themselves into the free-swimming 

 bell-shaped organisms in which they were actually produced, but 



. 33.— Free-swiinminpr mediisifonn gnnophore of liovgaini'Ulm superdliaris, 

 a fixed Hydroid. Enlarged (after A. Agassiz). 



into the plant-like, rooted, and compound zoophj'te, from which the 

 generative buds were originally given forth. These free-swimming 

 bell-shaped reproductive buds or gonophores (fig. 33\ as we shall 

 see, are structurally identical with the smaller foi-ms of the so-called 

 Sea-jellies or Jfeilnmr ; and it is now known that most of these 

 Mediisce, though originally described as distinct beings, are really 

 nothing more than the free generative buds of the fixed Hydriizoa. 

 Hence, these free reproductive buds are usually spoken of as " me- 

 dusiform gonophores.'' We have here, then, an instance of what has 

 been, not quite appropriately, called "alternation of generations." 



