The turbellaria, doubtless, have the sense of smell, although we can
discover no special olfactory organ. This sense would seem to be as
old as protoplasm itself.
This distribution of the eyes around a large portion of the margin,
and certain other characteristics of the adult structure and of the
embryonic development, are very interesting, as giving hints of the
development of the turbellaria from some radiate ancestor. The mouth
is in a most unfavorable position, in or near the middle of the
body, rarely at the front end, as the animal has to swim over its
food before it can grasp it. The animal only slowly rids itself of
old disadvantageous form and structure and adapts itself completely
to a higher mode of life.
By far the most highly developed system in the body is the
reproductive. It is doubtful whether any animal, except, perhaps,
the mollusk, has as complicated and highly developed reproductive
organs. By markedly higher forms they certainly grow simpler.
And here we must notice certain general considerations. We found
that reproduction in the amoeba could be defined as growth beyond
the limit normal to the individual. This form of growth benefits
especially the species. The needs and expenses of the individual
will therefore first be met and then the balance be devoted to
reproduction.
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