Page:Mind (Old Series) Volume 9.djvu/428

 416 D'ARCY w. THOMPSON: Other Coelenterates, Actiniae and Medusae, show the same power of recovery of lost parts, or of being multiplied by artificial section : and the practice of cutting up living Sponges and placing the pieces again in the sea has been recommended as a valuable means of multiplying commercial species. The power of regeneration in Echinodernis is as strongly marked as in Coelenterates. The halves of a Starfish torn in two develop new arms, and become completely restored ; while even a single amputated arm buds out a body and new arms, and be- comes an entire and perfect starfish. If the disc of an Ophiurid be torn across, the halves become similarly regenerated, but the single arm of an ophiurid cannot develop into an entire animal. The diverticulum of the gut, present in the Starfish arm, but not in that of the Ophiurid, seems to be necessary to effect the nutri- tion of the part during the process of restorative growth. In like manner, a single amputated tentacle of Hydra is not able to reproduce the body of the animal. In Echinoderms we find the capability for regeneration coupled commonly with the habit of voluntary fission. Every collector knows how hard it is to pre- vent live Brittle-stars from falling to pieces, and a well-known and characteristic passage in Edward Forbes's British Starfishes describes the same inconvenient tendency in Linckia. In the last-named and some other genera Kowalevsky, Simroth and others have described fission as a usual mode of reproduction. In Worms of many different families, like phenomena occur, though here our knowledge is limited to the effects of transverse fission. Nearly 150 years ago, Bonnet instituted experiments on an Oligochaete worm, Liunijriculus variegatus, which have been recently repeated and extended by Dr. C. Biilow. 1 Even a few segments of this worm, taken from the middle of the body, and possessing neither mouth nor brain nor sense-organs nor anal aperture, develop into a perfect worm provided with all those organs. And the new worm is just as safely susceptible of di- vision as the old. Biilow has also shown that Lumbriculus reproduces by asexual fission under natural conditions. Ee- generation-experiments by Biilow on Gephyrea (Aspidosiphon and Phascolosma) are described in the Biologisches Centmlblatt for March 1, 1883. In these worms the mouth, tentacles, hooks and circum-cesophageal nerve-ring were found to be quickly restored after amputation. Zeppelin has carefully described the mode of fission in Cteno- drilus. 2 In this worm a constriction appears, which deepens until the animal is cut in two ; and it is not till some time afterwards that the head of the one individual or the tail of the other begins to develop. The case is different in several polychaetous Annelids, e.ij., Myrianida, where a median body-segment develops eyes, 1 Zeitsch.f. vriss. Zool., xxxix., pp. 64-96, 1883. Arch. f. Naturgesch., xlix. 2 Zool. Anzeiger, vi., No. 130, 1883.