DEGENERATION THROUGH QUIESCENCE
While parasitism is the principal cause of degeneration among animals, yet
it is not the sole cause. It is evident that if for any other reason
animals should become fixed, and live inactive lives, they would
degenerate. There are not a few instances of degeneration due simply to a
quiescent life, unaccompanied by parasitism.
The Tunicata, or sea squirts, are animals which have become simple through
degeneration, due to the adoption of a sedentary life, the withdrawal from
the crowd of animals and from the struggle which it necessitates. The
young tunicate is a free-swimming, active, tadpolelike, or fishlike
creature, which possesses organs very like those of the adult of the
simplest fishes or fishlike forms. That is, the sea squirt begins life as
a primitively simple vertebrate. It possesses in its larval stage a
notochord, the delicate structure which precedes the formation of a
backbone, extending along the upper part of the body below the spinal
cord. The other organs of the young tunicate are all of vertebral type.
But the young sea squirt passes a period of active and free life as a
little fish, after which it settles down and attaches itself to a shell or
wooden pier by means of suckers, and remains for the rest of its life
fixed. Instead of going on and developing into a fishlike creature, it
loses its notochord, its special sense organs, and other organs; it loses
its complexity and high organization, and becomes a "mere rooted bag with
a double neck," a thoroughly degenerate animal.
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