PEDIGREE OF STAR-FISHES

Clypeastridæ
Spatangidæ│
Dysasteridæ│
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Aspidochirota│
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Synaptida│
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Cassidulidæ
Petalosticha
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Liodermatida
Apodia		│
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Echinonidæ
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Galeritidæ│
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Echinometridæ│
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Dendrochirota
Eupodia
Holothuriæ		SalenidæLatistellæ

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Angustistellæ
Desmosticha
Autechinida
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 Colocrinæ
Phytastra│
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Sphæronitidæ│
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Eocidaridæ│
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Eleutherocrina
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Ophiastra
Discogastra		│
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Agelacrinæ
Cystidea		│
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Melonitida
Palechinida
Echinida		│
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Phatnocrinæ
Brahiata		│
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Brisingastra│
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Pentremitida
Blastoidea
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Colastra

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Brachiata
Crinoida
Crinastra
Tocastra
Actinogastra
Asterida
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Phracthelminthes
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Cœlomati
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Gastræa

Hence the Star-fishes would be compound stocks of worms which, by the radial formation of buds, have developed out of true articulated worms, or Annelids. This hypothesis is most strongly supported by the comparative anatomy, and by the ontogeny of some Star-fishes (Colastra), and of segmented worms. The many-jointed Ring-worms (Annelida) in their inner structure are closely allied to the individual arms or radii of the Star-fishes, that is to the original single worms, which each arm represents. Each of the five worms of the Star-fish is a chain composed of a great number of equi-formal members, or metamera, lying one behind the other, like every segmented Worm, and every Arthropod. As in the latter a central nervous cord, the ventral nerve cord runs along the central line of the ventral wall of each segment. On each metameron there is a pair of non-jointed feet, and besides these, in most cases, one or more hard thorns or bristles similar to those of many Ring-worms. A detached arm of a Star-fish can lead an independent life, and can then, by the radially-directed growth of buds at one end, again become a complete star.

The most important proofs, however, of the truth of my hypothesis are furnished by the ontogeny or the individual development of the Echinoderma. The most remarkable facts of this ontogeny were first discovered in the year 1848 by the great zoologist, Johannes Müller of Berlin. Some of its most important stages are represented on Plates [VIII]. and [IX]. (Compare their explanation in the Appendix.) Fig. A on Plate [IX]. shows us a common Sea-star (Uraster), Fig. B, a Sea-lily (Comatula), Fig. C, a Sea-urchin (Echinus), and Fig. D, a Sea-cucumber (Synapta). In spite of the extraordinary difference of form manifested by these four representatives of the different classes of Star-fishes, yet the beginning of their development is identical in all cases. Out of the egg an animal-form develops which is utterly different from the fully developed Star-fish, but very like the ciliated larvæ of certain segmented Worms (Star-worms and Ring-worms). This peculiar animal-form is generally called the “larva,” but more correctly the “nurse” of these Star-fish. It is very small and transparent, swims about by means of a fringe of cilia, and is always composed of two equal symmetrical halves or sides. The fully grown Echinoderm, however—which is frequently more than a hundred times larger, and quite opaque—creeps at the bottom of the sea, and is always composed of at least five co-ordinate pieces, or antimera, in the form of radii. Plate [VIII]. shows the development of the “nurses” of the four Echinoderms represented on Plate [IX].

The fully developed Echinoderm arises by a very remarkable process of budding in the interior of the “nurse,” of which it retains little more than the stomach. The nurse, erroneously called the “larva,” of the Echinoderm, must accordingly be regarded as a solitary worm, which by internal budding produces a second generation, in the form of a stock of star-shaped and connected worms. The whole of this process is a genuine alternation of generations, or metagenesis, not a “metamorphosis,” as is generally though erroneously stated. A similar alternation of generations also occurs in many other worms, especially in some star worms (Sipunculidæ), and cord worms (Nemertinæ). Now if, bearing in mind the fundamental law of biogeny, we refer the ontogeny of Echinoderma to their phylogeny, then the whole historical development of the Star-fishes suddenly becomes clear and intelligible to us, whereas without this hypothesis it remains an insoluble mystery. (Compare Gen. Morph. ii pp. 95-99.)

Besides the reasons mentioned, there are many other facts (principally from the comparative anatomy of Echinoderma) which most distinctly prove the correctness of my hypothesis. I established this hypothesis in 1866, without having any idea that fossil articulated worms still existed, apparently answering to the hypothetical primary forms. Such have in the mean time, however, really been discovered. In a treatise “On the Equivalent of the North American Taconic Schist in Germany,”[3] Geinitz and Liebe, in 1867, have described a number of articulated Silurian worms, which completely confirm my suppositions. Numbers of these very remarkable worms are found in an excellent state of preservation in the slates of Würzbach, in the upper districts of Reusz. They are of the same structure as the articulated arm of a Star-fish, and evidently possessed a hard coat of mail, a much denser, more solid cutaneous skeleton than other worms in general. The number of body-segments, or metamera, is very considerable, so that the worms, although no more than a quarter or half an inch in breadth, attained a length of from two to three feet. The excellently preserved impressions, especially those of the Phyllodocites thuringiacus and Crossopodia Henrici, are so like the arms of many Star-fish (Colastra) that their true blood relationship seems very probable. This primæval group of worms, which are most probably the ancestors of Star-fish, I call Mailed worms (Phracthelminthes, p. [150].)

STAR FISHES. FIRST GENERATION. WORM PERSON. Pl. viii.