REGENERATION, in zoology, signifies
the reproduction, or natural restoration, of parts
of the body, whether external or internal, lost
by injury. This power of renewal belongs in
some degree to all living creatures, and is one
of the distinguishing features of organic, as
distinct from inorganic, nature. A broken
crystal may rebuild itself when it is allowed to
remain for a suitable time in a saturated
solution of the mineral of which it consists; but this
renewal is accomplished wholly by external
accretion of new material, whereas an animal
supplies a lost part wholly by the ordinary process
of food-assimilation and growth.
This power is most manifest, as might be
predicted, in the animals of the simplest
organization, and in those most exposed to serious
mutilation, and decreases in a varying scale as
organisms become more and more complex, until in the higher vertebrates (except lizards) it is limited to the healing of wounds. (The success of what surgeons call grafting, that is, the organic attachment of new flesh to old, as when detached skin is placed over a flayed surface, might be classified here, but it is an artificial utilization of the regenerative principle).
Among such lowly animals as protozoans, cœlenterates, echinoderms and worms, the power of regeneration is very great and of much importance, although it appears with much inequality and with some strange irregularities; among crustaceans it is strong in some groups and not in others; and among the vertebrates is confined mainly to the amphibians and the lizards.
As long ago as the middle of the 18th
century it was learned that if a hydra be sliced
into thin cross-sections from each ring would
sprout a fringe of tentacles, and when cut
lengthwise into strips each strip would form
for itself a complete hydra. The same thing
happens when protozoans, and even some
worms, are dissected. “If,” says Morgan, “a
fresh-water worm (Lumbriculus) is cut into
pieces, each piece makes a new head at its
anterior end, and a new tail at the posterior
end; in this way as many new worms are
produced as there are pieces.” Not all worms are
so energetic. Thus in the common earthworm
the cutting must be done carefully. “If from
one to five of the anterior segments be cut off,” according to Packard, “the same number comes back; if more are cut off the process of regeneration begins only after a longer interval, and only four or five segments come back as a rule; if the cut be behind the middle . . . fewer worms succeed in regenerating at all.” In the case of another annelid (Allolobophora) allied to the earthworm, when cut in two in the middle the posterior piece "regenerates at its anterior cut end not a head but a tail." This is one of the many curious examples of inconsistency or irregularity in this function. In the hydroids the presence or absence of light has much to do with the form regeneration will take, and elsewhere other external conditions are influential.
Among the echinoderms regeneration is
most conspicuous is the starfishes. Before this
ability was generally known the oystermen of
Long Island Sound, whose planted oyster-beds
were infested with destructive starfishes, used
to catch great numbers of them by dragging
“tangles”; piling them on the deck of the boat
they would, as an easy means of dispatching
them quickly, saw through the heaps by pulling
a coarse cord and then throw the severed halves
overboard. The result was that each half of
the starfish (unless its centre was utterly
ruined) put forth new arms, and thus the
number of pests was doubled instead of
decreased. Now the starfishes caught are taken
ashore and turned into fertilizer.