(>i INVERTEBEATE ANIMALS. 



6), from its resemblance in shape to the Fresh-water 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-tuba becomes 

 elongated and exhibits a number of transverse grooves. These 

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

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

 saucers placed one above the other (fig. 42, c). 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. 42, d) drop off, one by one, and present them- 

 selves in the form of independent free-swimming Medusae. These 

 reproductive zooids or Medusae eat voraciously, and increase rapidly 

 in size, becoming not only comparatively, but often actually, gigantic. 

 Thus, in one case the reproductive zooid has been known to attain 

 a size of seven feet across, with tentacles one hundred feet or more 

 in length, though the fixed 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 Hydra- tuba, 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-blubbers which 

 abound around most coasts in svimmer are reajly the detached repro- 

 ductive buds of minute fixed Hydrozoa; and it may be as well to men- 

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

 they may be distinguished from the smaller or naked-eyed Mediisce, 

 to which they have a decided superficial likeness. In the commonest 

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

 Cyanea), the body consists of a great bell-shaped gelatinous disc or 

 "umbrella" (fig. 4.3), from the roof of which is suspended a single 

 polypite, the lips of which are extended into lobed processes, often 

 extending far below the margin of the disc. The digestive cavity 

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

 ing gastro-vascular canals, which proceed towards the margin of the 

 umbrella. These radiating canals are never less than eight in num- 

 ber, and on their way to the margin of the disc they break up into 

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

 form a complicated network. At the margin of the bell they all 

 open into a circular vessel, which in turn sends processes into a 



