Evolution of life / by Henry C. Chapman.

  • Henry Cadwalader Chapman
Date:
1873
Catalogue details

Licence: Public Domain Mark

Credit: Evolution of life / by Henry C. Chapman. Source: Wellcome Collection.

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    stationary condition, rooted by a stern ; in this immature state it was supposed for a long time to be a distinct animal, known as the Pentacrinus. (Fig. 42.) After a time, how- ever, tlie stem disappears, and the little creature floats off as the very pretty Comatula. The Comatula is very inter- esting, as its early Pentacrinus stage is the only living representative of the Crinoids, known commonly as lily stones, and as St. Cuthbert’s beads, when segments of the stem alone are preserved. The Crinoids are now extinct but are preserved in great profusion in the fossil state. As the star-fishes in one stage of their existence are more or less fixed, and as the Crinoids have died out, save the only living example, the young of the Comatula, it is possible that the Crinoids are the earliest of the Echinodermata. Haeckel, however, considers the Crinoids as a very ancient offshoot of the star-fishes, adapted to the fixed state of living. Per- haps the Crinoids and star-fishes are the diverging stems of an intermediate group, partaking in its nature of the peculiarities of both these classes. In either case the star- fishes are the progenitors of the sea-urchins, and they of the sea-cucumbers. Imagine the five arms of the star-fish bend- ing down until their free ends, uniting in the middle, form a ball-shaped figure; suppose the empty spaces between the arms to be filled up, and a sphere will be formed. Such are the relations of the star-fish to the sea-urchin, or Echinus. Many intermediate fossil forms have been found connecting these extremes. The sea-urchins, or sea-eggs, are covered with innumerable spines or bristles; hence their name of Echini. (Fig. 44.) These spines are movable, being loosely articulated to little knobs covering the body. When one watches a living Echinus, there may be seen protruding between the spines sucker-like appendages, which serve as a means of progression. If the spines be removed, the body of the Echinus is seen to be a hollow sphere, com- posed of arms (ambulacral plates) and intermediate arms
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