REGENERATION, is 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 solu tion of the mineral of which it consists; but this renewal is accomplished wholly by external ac cretion of new material, whereas an animal supplies a lost part wholly by the ordinary proc ess of food-assimilation and growth.
This power is most manifest, as might be predicted, in the animals of the simplest or ganization, 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 sur face, might be classified here, but it is an artificial utilization of the regenerative prin ciple).
Among such lowly animals as protozoans, coelenterates, echinoderms and worms, the power of regeneration is' very great and of much importance, although it appears with much inequality and with some strange irregu larities; among crustaceans it is strong in some groups and not in others; and among the verte brates is confined mainly to the amphibians and the lizards.
As long ago as the middle of the 18th cen tury 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 pro duced 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 regen eration 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 (Allolobo phora) allied to the earthworm, when cut in two in the middle the posterior • piece °regen erates at its anterior cut end not a head but a tail.* This is one of the many curious ex
amples of inconsistency or irregularity in this function. In the hydroids the presence or ab sence 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 oystertnen 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 de creased. Now the starfishes caught are taken ashore and turned into fertilizer.
Crabs and other crustaceans will develop new limbs to replace lost ones. °If," says Calman ((Life of Crustacea,' 1911), °a lobster be caught by one of its claws or by a leg, it very readily parts with the limb in its struggles to escapes ; and he explains that the connection at the joint of the second and third segments is slight, and that there is only a small hole through which blood vessels and nerves pass to the distal segments, and this is quickly plugged by a blood-clot. "Beneath the scar which forms on the stump of a separated limb a sort of bud grows, and gradually assumes the form of the lost segments . . . and ultimately provides in normal cases a new member, similar in every detail to that which has been lost* Amphibians, especially salamanders and the young frogs, suffer greatly from mutilation, not only by enemies who catch them by the tail or a limb, which may be lost in the effort to escape (in some cases by voluntary slough ing off of tail or gills), but by the biting of their own fellows, especially when many larva are associated. Salamanders are particularly subject to loss or mutilation of their long tails. These, and lost hands, are restored in the course of three or four months; but in place of the bones that formerly existed in the part a similar cartilaginous framework only is sup plied. Fully grown frogs are slower than the young and often the restoration is incomplete.