242 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 

 bo different from the animal that gives it birth 

 that one would suppose the Hydroid parent urns' 



* See wood-cut, p. 240. 



