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Of the free echinoderms, such as the starfish and the sea urchin, the former has been found in the Cambrian rocks of Europe, but neither have so far been discovered in the strata of this period in North America.
ECHINODERMS. The cystoid reaches its climax, but there appear now two higher types of echinoderms, the crinoid and the starfish. The CRINOID, named from its resemblance to the lily, is like the cystoid in many respects, but has a longer stem and supports a crown of plumose arms. Stirring the water with these arms, it creates currents by which particles of food are wafted to its mouth.
ECHINODERMS. This subkingdom comprises at present such familiar forms as the crinoid, the starfish, and the sea urchin. The structure of echinoderms is radiate. Their integument is hardened with plates or particles of carbonate of lime.
Deaths will occasionally take place; and even suicide is said to be resorted to by the wicked family of the Echinoderms. To procure specimens for the aquarium requires some knack and knowledge. The sea-shore must be haunted, and even the deep sea explored. At the extreme low-water of new or full moon tides, the rocks and tide-pools are to be zealously hunted over by the aquarian naturalist.
Yet radical and sweeping as the changes of organism above described must be, we do not feel them to be more a bar to personal identity than the considerable changes which take place in the structure of our own bodies between youth and old age. Perhaps the most striking illustration of this is to be found in the case of some Echinoderms, concerning which Mr.
I trust I have succeeded in showing that the three Orders of the Acalephs are, like the five Orders of the Echinoderms, different degrees of complication of the same structure.
With regard to organisms, however, the wonderful Acanthometræ and the Polycystina may be mentioned as presenting complexities of form which can hardly be thought to be due to other than internal causes. The same may be said of the great group of Echinoderms, with their amazing variety of component parts.
These remarkable hyper-metamorphoses remind us of the metamorphosis of the embryo of Echinoderms into the Pluteus-and Bipinnaria-forms of the starfish, sea urchins and Holothurians; of the Actinotrocha-form larva of the Sipunculoid worms; of the Tornaria into Balanoglossus, the worm; of the Cercaria-form larva of Distoma; of the Pilidium-form larva of Nemertes; and the larval forms of the leeches; as well as the mite Pentastomum, and certain other aberrant mites, such as Myobia.
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