249 ALTERNATE GENERATIONS. 
of the stem sideways, which elongates gradually, 
putting out tentacles as it grows longer, while 
at the upper end‘an aperture is formed to make 
the mouth. This is one of the lower group 
of Radiates, known as Hydroids, and long be- 
lieved to be Polyps, from their mode of living 
in communities and reproducing their kind by 
budding, after the fashion of Corals. 
But if such a little tuft of Hydroids has been 
gathered in spring, a close observer may have 
an opportunity of watching the growth of an- 
other kind of individual from it, which would 
seem to show its alliance with the Acalephs 
rather than the Polyps. At any time late in 
February or early in March, bulb-like projec- 
tions, more globular than the somewhat elon- 
gated buds of the true Hydroid heads, may be 
seen growing either among the tentacles of one 
of these little animals, or just below the head 
where it merges in the stem.* Very delicate 
and transparent in substance, it is hardly. per- 
ceptible at first; and the gradual formation of 
its internal structure is the less easily discerned, 
because a horny sheath, forming the outer cover- 
ing of the Hydroid stock, extends to enclose and 
shield the new-comer, whom we shall see to be 
so different from the animal that gives it birth 
that one would suppose the Hydroid parent must 
* See wood-cut;, p. 240. 
