LUCERNARIDA AND GRAPTOLITIDA. 85 
with which its surface is covered (fig. 28, a), This little body, 
on finding a suitable locality, fixes itself by one: end, and de- 
velops a mouth and tentacles at the other, when it is known 
as a “ Hydra-tuba” (fig. 28, 4), 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, whilst it is incapable 
of developing the essential elements of reproduction. Under 
certain circumstances, however, reproductive zodids are pro- 
duced by the following singular process: The Hydra-tuba be- 
comes elongated and exhibits a number of transverse grooves 
(fig. 28, c). These 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 the 
other (fig. 28, @). The tentacles now disappear, and a fresh 
circle is formed close to the base of the Hydra-tuba (fig. 28, ¢). 
Finally, all the saucer-like segments above the new circle of 
tentacles drop off one by one, and present themselves in the 
form of independent free-swimming J/eduse (fig. 28, f). These 
reproductive zodids or AZeduse@ eat voraciously, and increase 
rapidly in size, becoming not only comparatively, but often 
actually, gigantic. Thus, in one case the reproductive zodid 
has been known to attain a size of seven feet across, with ten- 
tacles fifty feet in length, though the fixed organism from which 
it was produced was no more than half an inch in height. These 
gigantic reproductive 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 de- 
tached. 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 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 struc- 
ture, and the points by which they may be distinguished from 
the smaller or naked-eyed Jleduse@, to which they have a 
decided superficial likeness. In the commonest forms of these 
zodids (such as our familiar sea-blubbers, Aurelia and Cyanea), 
the body consists of a great bell-shaped gelatinous disc or 
