Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/57

Rh the union of five worms, the worms having united at their posterior ends, since the eyes are seen at the free ends of the star-fish. This interpretation of the structure of the star-fish is not without a parallel among the worms. The Botryllus, one of the sac-worms, is really composed of many little Ascidians living as one individual. There is nothing more extraordinary in five worms living together as a star-fish, than in many little Ascidians living together as a Botryllus. This view of the origin and the structure of the star-fish, first proposed by Haeckel, is in perfect harmony, according to the same author, with the facts of its development. The egg of the star-fish is transformed into a larva, provided with an intestine from the inner part of the body of the larva. Around its mouth appear five distinct layers, which, uniting at their posterior ends, form the body and arms of the mature animal. The same kind of reproduction is seen in the Sipunculi, which are supposed to be indirectly the ancestors of the star-fish, and also in the Nemertian worms, from which, or their allies, the Sipunculi and other articulated worms have descended. Within a few years there have been found a very well-preserved group of fossil worms,—the Phractelminthes, or mailed worms. These are considered by Haeckel to be intermediate between the Sipunculus and the star-fish, they being scarcely distinguishable from the arms of the latter. Through the union of worms, like the Phractelminthes, have the star-fishes been produced. The origin of the Asteridae, or star-fishes, from the worms, is in perfect harmony with the structure, development, and petrified remains of the group. The most striking facts of their economy are explainable on such a theory, but are perfectly meaningless on any other. The star-fishes are probably the ancestors of the remaining Echinodermata. Passing over the Ophiuridae, which differ but little from the star-fishes, we come to the Feather-stars, or Comatula (Fig. 43), which, when young, live in a