The reproductive organs for the production of eggs and spermatozoa
form little protuberances on the outside of the body below the
tentacles. But hydra reproduces mostly by budding; new individuals
growing out of the side of the old one, like branches from the trunk
of a tree, but afterward breaking free and leading an independent
life. There are special forms of cells besides those described;
nettle cells for capturing food, interstitial cells, etc., but these
do not concern us.
The distance from the single-celled amoeba to hydra is vast,
probably really greater than that between any other successive terms
of our series. It may therefore be useful to consider one or two
intermediate forms and the parallel embryonic stages of higher
animals, and to see how the higher many-celled animal originates
from the unicellular stage.
The amoeba is an illustration of a great kingdom of similar,
practically unicellular forms, which have played no unimportant part
in the geological history of the globe. These are the protozoa. They
include, first of all, the foraminifera, which usually have shells
composed of carbonate of lime. These shells, settling to the bottom
of the ocean, have accumulated in vast beds, and when compacted and
raised above the surface, form chalk, limestone, or marble,
according to the degree and mode of their hardening.
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