64 INVERTEBRATE ANIMALS. 



the roof of this disk there is suspended a kind of handle, 

 which corresponds to the clapper of the bell, and is termed 

 the " manubrium " (Lat. for handle). From the fixed or proxi- 

 mal extremity of the central process or manubrium proceed 

 four canals, which extend to the margin of the beU, where 

 they all open into a circular canal surrounding the mouth 

 of the bell. This bell-shaped reproductive bud may attain no 

 higher development than this, and may remain permanently 

 attached to the parent organism from which it is produced. 

 In other cases, however, a higher state of development is 

 reached (Fig. 16, d). The generative bud or gonophore be- 

 comes detached from its parent colony; the manubrium or 

 central process develops a mouth at its free extremity; the 

 mouth of the bell becomes partially closed by an inward pro- 

 longation or shelf, called the " veil ; " and a series of ten- 

 tacles are developed from its margin. The generative bud, 

 thus liberated, leads a wholly independent existence. The 

 manubrium, having developed a mouth, assumes the func- 

 tions of a true polypite, and its cavity acts as a digestive 

 sac. The whole organism swims about freel}"^, and has the 

 power of assimilating food, and thus of attaining to a com- 

 paratively gigantic size. This independent existence, how- 

 ever, only goes on till such time as the elements of reproduc- 

 tion can be produced. The ova and sperm-cells are developed 

 in specialized portions of this generative bud, and then it 

 ceases to exist. The ova, however, when fertilized, do not 

 develop themselves into the free-swimming bell-shaped or- 

 ganisms in which they were actually produced, but into the 

 plant-like, rooted, and compound zoophyte, from which the 

 generative buds were originally given forth. These free- 

 swimming bell-shaped reproductive buds or gonophores, as 

 we shall see, are identical structurally with the smaller forms 

 of the so-called sea-jellies or Medusm ; and it is now known 

 that most if not all of these Medusm, though originally de- 

 scribed as distinct beings, are really nothing more than the 

 free generative buds of the fixed Hydrozoa. We have here, 

 then, an instance of what has been not quite appropriately 

 called " alternation of generations." We have a compound 

 fixed animal, in many respects comparable to a plant, pro- 

 ducing a special series of buds which are devoted to the pro- 

 cess of reproduction. These buds are cast off as independent 

 beings to lead an independent life, and they are furnished 

 with the necessary organs to preserve their existence till they 

 are able to mature the reproductive elements. When once 



