The body is here only a vehicle for ova.
Furthermore, in volvox there has arisen such an interdependence of
cells that we can no longer speak of it as a colony. The colony has
become an individual by division of labor and the resulting
differentiation in structure.
But hydra gives us but a poor idea of the coelenterata, to which
kingdom it belongs. The higher coelenterata have nearly or quite
all the tissues of higher animals--muscular, connective, glandular,
etc. And by tissues we mean groups of cells modified in form and
structure for the performance of a special work or function. The
protozoa developed the cell for all time to come, the coelenterata
developed the tissues which still compose our bodies. But they had
them mainly in a diffuse form. A sort of digestive and reproductive
system they did possess. But the work of arranging these tissues and
condensing them into compact organs was to be done by the next
higher group, the worms.
Let us now take a glance at certain stages of embryonic development
which correspond to these earliest ancestral forms. We should expect
some such correspondence from the fact already stated that the
embryonic development of the individual is a brief recapitulation of
the ancestral development of the species or larger group. The egg of
the lowest vertebrate, amphioxus, shows these changes in a simple
and apparently primitive form.
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