64 



IXVEIiTEBKATE ANIMALS. 



bell becomes partially closed by au 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 exi.st. The ova, however, 

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

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



FiLj. 33. — Frei'-swiimnins Tiipflnsiform gnnnphnrn of BniigainriJJrfi svp'^reilinris, 

 n ti\i:i\ I-Iyaioi.l. Enlal■,^'0.1 (aftrr A. Agassiz), 



into the plant-like, rooted, and cnmpoinid zoophyte, from which the 

 generative buds were originally given forth. These free-s.winnning 

 bell-shaped ro]iroductive Imds or gonophores (fig. 33), as we shall 

 see, are structinally identical with the smaller forms of the so-called 

 Seii-jellies or ^fribixir : ;uid it is now known that most of these 

 jVi'(/iix(v, though origin.ally desia-ilied as distinct liciugs, are really 

 nothing more tliau the fii'c generative Imds of the fixed Ifi/drozoa. 

 Hence, these free reproductive l>u(ls are usuallv s|)oken of as "me- 

 dusiform gonn|>hiires." Wo have here, then, an instance of what has 

 been, not ijuite ap|)ro]iriately, called "alternation of generations." 



