80 INVERTEBRATE ANIMALS. 



This little body, on finding a suitable locality, fixes itself by 

 one end, and develops a mouth and tentacles at the other, 

 when it is known as a "Hydra-tuba" (Fig. 23, #), 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 

 possesses the power of forming large colonies by gemmation, 

 while it is incapable of developing the essential elements of 

 reproduction. Under certain circumstances, however, repro- 

 ductive zoftids are produced by the following singular process : 

 The Hydra-tuba becomes elongated, and exhibits a number 

 of transverse grooves (Fig. 23, c). These grooves go on get- 

 ting deeper and 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. 23, d). The tentacles now 

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

 Hydra-tuba (Fig. 23, e). Finally, all the saucer-like segments 

 above the new circle of tentacles drop off one by one, and pre- 

 sent themselves in the form of independent, free-swimming 

 Medusae, (Fig. 23,/). 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 zoo'id has been known to attain a 

 size of seven feet across, with tentacles fifty feet in length, 

 though the fixed organism from which it was produced, w r as 

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

 ductive bodies live an independent life until they are able to 

 produce ova and sperm-cells, when they die. The fertilized 

 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 " alterna- 

 tion of generations." 



It is now known, then, that most of the great sea-blubbers 

 which abound around our coasts in summer are really the 

 detached reproductive buds of minute fixed Hydrozoa ; and it 

 may be as well to mention the leading features in their struct- 

 ure, and the points by which they may be distinguished from 

 the smaller or naked-eyed Medusae, to which they have a de- 

 cided superficial likeness. In the commonest forms of these 

 zooids (such as the familiar sea-blubbers, Aurelia and Cyaned)^ 

 the body consists of a great bell-shaped gelatinous disk or 

 " umbrella," 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 disk (Fig. 24). 



