The Zoophytes. 307 
This curious alternation of generations, a phenomenon 
perfectly distinct from the early embryonic changes of 
the organism, finds a very close parallel in the Vegetable 
World among the ferns and allied plants, where the 
familiar leafy growth, like the plant-like Hydroid colony, 
is destitute of the power of producing sexual elements, 
and produces instead an asexual generation in the form 
of minute buds or spores (generally to be noticed in 
groups on the back of the leaves or fronds), which fall 
away and produce, under suitable conditions of tempera- 
ture and moisture, a leaf -like expansion on which the true 
sexual elements develop ; and these sexual elements 
give rise, not to a leaf-like expansion like that on 
which they were produced, but to the common leafy 
plant which again starts the asexual generation of the 
series. 
In connection with the jelly-fishes, it is a curious and 
noteworthy fact, that several forms, and these among 
the largest, are known which have no Hydroid stage, this 
condition having evidently been lost ; so that here the 
sexual elements of the medusa give, rise to other medusae 
in direct succession. 
The hard or skeletal tissues of the Zoophytes are 
considerably varied, and take on one of three chief forms 
according to the special group in which they are ex- 
amined. In the group of the Hydra-like animals 
(Hydrozoa), among the plant-like Hydroid Zoophytes, 
with the exception of a few forms having a calcareous 
skeleton covered by a fleshy layer, a horny deposit covers 
the greater part of the colony, extending at the base as 
well over the rootlike filaments which fix the growth to 
foreign objects, as over the stem-like portion as a sup- 
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