The formation of the new part takes place somewhat differently, according to Schultz, when the leg is amputated between two segments than when cut off at the coxa. In the latter case, there is produced from the cut-end of the last segment a solid rod which, as it grows longer, bends on itself several times. Joints appear in the rod, beginning at the base. The leg is set free at the next moult. If the leg is cut off nearer the distal end a smaller rod is formed, that extends straight forward, or may be thrown into a series of folds. It lies, however, inside of the last segment, since the surface exposed by the cut is quickly covered over by a chitinous covering. The piece is set free at the next moult.
Loeb has found that if the body of the pycnogonid, Phoxichilidium maxillare, is cut in two there regenerates from the posterior end of the anterior half a new body-like outgrowth.
Without attempting to describe the many cases in worms and mollusks in which there is no obvious connection between the power of the part to regenerate and its liability to injury, but where it is more difficult to show that it may not exist, let us pass to an examination of the regeneration of the starfish. It has been known since the time of Réaumur that starfish have the power of regenerating new arms if the old ones are lost. It has been stated that in certain starfishes an arm itself can produce a new starfish,—Haeckel (’78), P. and R. Sarasin (’88), von Martens (’84), and Sars (’75),—but this has been denied by other observers. In several species of starfishes, the separated arm does not regenerate; but if a portion, even a small piece, of the disk is left with the arm, a new disk and arms may develop ([Fig. 38], F). When the arm of Asterias vulgaris is injured it pinches off in many cases at its base, and a new arm grows out from the short stump that remains. When these starfishes regenerate new arms in their natural environment, the new arms almost always arise from this breaking region.[45] Thus King found out of 1914 individuals of Asterias vulgaris collected at random, 206, or 10.7 per cent, had one or more new arms, and all these except one arose from near the disk. In other species it appears that the outer portions of the arm may be broken off without the rest of the arm being thrown off. King has found that in asterias, regeneration takes place more rapidly from the base than at a more distal level. It may appear, at first thought, that the more rapid regeneration of the arm at the place at which it is usually thrown off may be associated with its more frequent loss at this region—in other words, that the more rapid regeneration has been acquired by the region at which the arm is generally broken off. This interpretation is, however, excluded by the fact that, in general, the nearer to the base the arm is cut off, so much the more rapid is its regeneration. In other words, the more rapid regeneration of the arm at the base is only a part of a general law that holds throughout the arm. If the proposition is reversed, and it is claimed that the arm has acquired the property of breaking off at the base, because it regenerates more rapidly at that level, the following fact recorded by King is of importance, viz. that, although the arm regenerates faster at the base, yet a new arm is not any sooner produced in this way, since there is more to be produced and the new arm from the base may never catch up to one growing less rapidly from a more distal cut-surface, but having a nearer goal to reach.
The results of our examination show that those forms that are liable to have certain parts of their bodies injured are able to regenerate not only these parts, but at the same time other parts of the body that are not subject to injury. The most remarkable instance of this sort is found in those animals having breaking-joints. In these forms, we find that regeneration takes place both proximal and distal to this region. If the power of regeneration is connected with the liability of a part to injury, this fact is inexplicable.
Turning now to the question as to whether regeneration takes place in those species that are subject to injury more frequently or better than in other species, we find that the data are not very complete or satisfactory for such an examination. It is not easy to ascertain to what extent different animals are exposed to injury. If we pass in review the main groups of the animal kingdom, we can at least glean some interesting facts in this connection.
In the protozoa nucleated pieces have been found to regenerate in all forms that have been examined, including amœba, difflugia, thalassicolla, paramœcium, stentor, and a number of other ciliate infusoria.
In the sponges it has been found by Oscar Schmidt that pieces may produce new individuals, but how widely this occurs in the group is not known. In the cœlenterates many forms are known to regenerate, and it is not improbable that in one way or another the process occurs throughout the group. The hydroid forms, hydra, tubularia, parypha, eudendrium, antennularia, hydractinia, podocoryne, etc., the jelly-fish, gonionemus, and certain members of the family Thaumantidæ, have been found to regenerate. Amongst the Scyphozoa, metridium, cerianthus, and the scyphistoma of aurelia regenerate, and the jelly-fishes belonging to this group have a limited amount of regenerative power.
In the platodes we find that all the triclads, thus far examined, including planaria, phagocata, dendrocœlum, and the land triclad, bipalium, regenerate. It has been shown that the marine triclads also regenerate, but less rapidly and extensively, while the marine polyclads have very limited powers of regeneration. The regeneration of the trematodes and cestodes has not, so far as I know, been studied, neither have the nematodes been examined from this point of view.
Some of the nemerteans regenerate, others do not seem to have this power. A small fresh-water form, tetrastemma, that I examined, did not regenerate, although some of the pieces, that were filled with eggs, remained alive for several months.
In the annelids we find a great many forms that regenerate—many marine polychæta have this power; all oligochæta that have been studied regenerate; both land forms, like lumbricus, allolobophora, etc., and fresh-water forms, like lumbriculus, nais, tubifex, etc.