y the aid
of which it swims about, the broader end in front. After a while it
attaches itself, not as might have been expected by the posterior but by
the anterior extremity (_2_). The cilia then disappear, a mouth is
formed at the free end, tentacles, first four (_3_), then eight, and at
length as many as thirty (_4_), are formed, and the little creature
resembles in essentials the freshwater polyp (Hydra) of our ponds.
[Illustration: Fig. 5.--Medusa aurita, and progressive stages of
development.]
At the same time transverse wrinkles (_4_) are formed round the body,
first near the free extremity and then gradually descending. They become
deeper and deeper, and develop lobes or divisions one under the other,
as at _5_. After a while the top ring (and subsequently the others one
by one) detaches itself, swims away, and gradually develops into a
Medusa (_6_). Thus, then, the life-history is very similar to that of
the Hydroids, only that while in the Hydroids the fixed condition is the
more permanent, and the free swimming more transitory, in the Medusae,
on the contrary, the fixed condition is apparently only a phase in the
production of the free swimming animal. In both the one and the other,
however, the egg gives rise not to one but to many mature animals.
Steenstrup has given to these curious phenomena, many other cases of
which occur among the lower animals, and to which he first called
attention, the name of alternations of generations.
In the life-history of Infusoria (so called because they swarm in most
animal or vegetable infusions) similar difficulties encounter us. The
little creatures, many of which are round or oval in form, from time to
time become constricted in the middle; the constriction becomes deeper
and deeper, and at length the two halves twist themselves apart and swim
away. In this case, therefore, there was one, and there are now two
exactly similar; but are these two individuals? They are not parent and
offspring--that is clear, for they are of the same age; nor are they
twins, for there is no parent. As already mentioned, we regard the
Caterpillar, Chrysalis, and Butterfly as stages in the life-history of
a single individual. But among Zoophytes, and even among some insects,
one larva often produces several mature forms. In some species these
mature forms remain attached to the larval stock, and we might be
disposed to regard the whole as one complex organism. But in others they
detach t
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