78 INVEr.TEBKATE AXIMALS. 



/)), from its resemblance in shape to the Fresh-watev Polype or Hydra. 

 The Hydra-tuba is only about half an inch in height, and it pos- 

 sesses the power of forming large colonies by gemmation, whilst it 

 is incapable of developing the essential elements of reproduction. 

 Under certain circumstances, however, reproductive zooids are pro- 

 duced by the following singular process : The Hydra-tnba becomes 

 elongated and exhibits a number of transverse grooves. Tliese 

 grooves go on getting deeper and deeper, and become lobed at their 

 margins, till the whole organism assumes the aspect of a pile of 

 saucers placed one above tlie other (fig. 42, <■). The tentacles now 

 disappear, and a fresh circle is formed close to the base of the 

 Hydra-tuba. Finally, all the saucer-like segments above the new 

 circle of tentacles (fig. 4:2, d) droji otf', one by one, and present them- 

 selves in the form of inilepeuilent free-swimming 2Iedus(e. These 

 reproductive zooids or Medusa eat voraciously, and increase rapidly 

 in size, becoming not only comparatively, l>nt often actually, gigantic. 

 Thus, in one case the reproductive zociid lias been known to attain 

 a size of seven feet acro-is, with tentacles one hun<lred feet or more 

 in length, though the ll.xed organism from which it was produced 

 was no more than half an inch in height. These gigantic reproduc- 

 tive bodies live an independent life until they are able to produce 

 ova and sperm-cells, when they die. The fertilised egg, however, 

 develops itself, not into the monstrous organism by which it -was 

 produced, but into the little fixed sexless Hydr;i-tul)a, from which the 

 generative bud was detached. We have, then, here another instance 

 of the so-called "alternation of generations." 



It is now known, then, that most of the great Sea-blubl>ers whicli 

 abound around most coasts in sunnner are really the detached repro- 

 ductive buds of minute Hxeil Hiidi-Dzou ; and it may be as well to men- 

 tion the leading features in their structure, and the noints by which 

 they may be distinguished from the smaller or uaived-eyed Medium\ 

 to whicli they hiive a decided superficial likeness. In the commonest 

 forms of these zooids (such as the common Sea-blubbers Aiirdki and 

 Cyanea), the body consists of a gTe.-it bell-shaped gelatinous disc or 

 "umbrella" (fig. 43), from the roof of which is suspended a single 

 ]iolypite, the lijis of which are extended into lolied processes, 'often 

 extending far lielow tlie margin of the (Use. The digestive cavity 

 of the polypite gives out from its upper extremity a series of radiat- 

 ing gastro-vascular canal.s, which ]>roceed towards the margin of tlio 

 umbrella. These ra,diating canals are never less than eight in num- 

 ber, and on their way to the margin of the di.sc they lireak up into 

 a, great nundier of smaller vessels, which unite with one another to 

 fo) ni a eoTuiilicated network. At the margin of the bell they all 

 opi.'ii illtd a cin'ular vesvl, which in turn sends processes into a 



