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With more or less modification in detail, the embryo has been observed to pass through these successive evolutional stages in sundry Sponges, Coelenterates, Worms, Echinoderms, Tunicates, Arthropods, Mollusks, and Vertebrates; and there are valid reasons for the belief that all animals of higher organisation than the Protozoa, agree in the general character of the early stages of their individual evolution.
Besides the star-fish there occur abundance of those peculiar bodies called Cystideae. They are the Sphaeronites of old authors, and were considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. With it have been found several other genera of the same family, such as Sphaeronites, Hemicosmites, etc.
The stemmed and lower division of the echinoderms was represented by a primitive type, the CYSTOID, so called from its saclike form, A small globular or ovate "calyx" of calcareous plates, with an aperture at the top for the mouth, inclosed the body of the animal, and was attached to the sea bottom by a short flexible stalk consisting of disks of carbonate of lime held together by a central ligament.
There is no a priori improbability in such a belief, for besides other annelids, especially the Arenicola marina, which throws up such a profusion of castings on our tidal sands, and which it is believed thus subsists, there are animals belonging to the most distinct classes, which do not burrow, but habitually swallow large quantities of sand; namely, the molluscan Onchidium and many Echinoderms.
Strange ball or cup-shaped little animals, with a hard frame, mounted on stony stalks and waving irregular arms to draw in the food-bearing water, are the earliest representatives of the Echinoderms.
Beside his duties as professor, he was printing at the same time his "Fossil Fishes," his "Fresh-Water Fishes," and his investigations on fossil Echinoderms and Mollusks, the illustrations for all these various works being under his daily supervision. The execution of these plates, under M. Nicolet's care, was admirable for the period.
The objection that broad statements of this kind, after all, rest largely on negative evidence is obvious, but it has less force than may at first be supposed; for, as might be expected from the circumstances of the case, we possess more abundant positive evidence regarding Fishes and marine Mollusks than respecting any other forms of animal life; and yet these offer us, through the whole range of geological time, no species ordinally distinct from those now living; while the far less numerous class of Echinoderms presents three, and the Crustacea two, such orders, though none of these come down later than the Palaeozoic age.
If I succeeded in explaining my subject clearly in the last article, my readers will have seen that the five Orders of the Echinoderms are but five expressions of the same idea; and I will now endeavor to show that the same identity of structural conception prevails also throughout the two other Classes of Radiates, and further, that not only the Orders within each Class, but the three Classes themselves, Echinoderms, Acalephs, and Polyps, bear the strictest comparison, founded upon close structural analysis, and are based upon one organic formula.
I should add, however, that many impressions described as produced by Worms are more likely to have been the tracks of Crustacea. But by far the most characteristic class of Articulates in ancient times were the Crustaceans. The Trilobites stand in the same relation to the modern Crustacea as the Crinoids do to the modern Echinoderms.
Among the Echinoderms we find that the Cystids and Blastoids have gone, and the sea-lilies reach their climax in beauty and organisation, to dwindle and almost disappear in the last part of the Mesozoic. One Jurassic sea-lily was found to have 600,000 distinct ossicles in its petrified frame. The free-moving Echinoderms are now in the ascendant, the sea-urchins being especially abundant.
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