| Fig. 47.—Palæaster Niagarensis (Hall). One of the oldest star fishes. | Fig. 48.—Palæchinus ellipticus (McCoy). One of the oldest types of sea-urchins. |
The most curious history in this group is that of the feather-stars. In the Early Cambrian they are represented by a few species known to us only in fragments, and these belong to a humble group (Cystideans) resembling the larval or immature condition of the higher Crinoids. [Fig. 49] shows one of these animals of somewhat later age. They have few or rudimentary arms and short stalks, and want the beautiful radial symmetry of the typical star-fishes. In the Silurian these creatures are reinforced by a vast number of beautiful and perfect feather-stars ([Figs. 50, 51]). These continue to increase in number and beauty, and apparently culminate in the Mesozoic, where gigantic forms exist, some of them probably having more complicated skeletons, in so far as number of distinct parts is concerned, than any other animals. Buckland has calculated that in a crinoid similar to that in [Fig. 52] there are no less than 150,000 little bones, and 300,000 contractile bundles of fibres to move them. In the modern seas the feather-stars have somewhat dwindled both in numbers and complexity, and are mostly confined to the depths of the ocean. On the other hand, the various types of ordinary star-fishes and sea-urchins have increased in number and importance. We thus find in this group a certain advance and improvement from the Cystideans of the Early Palæozoic to the sea-urchins and their allies. This advance is not, however, along one line for the Cystideans continue unimproved to the end. The Crinoids culminate in the Mesozoic, and are not known to give origin to anything higher. The star-fishes and sea-urchins commence independently, before the culmination of the Crinoids, and, though greatly increased in number and variety, still adhere very closely to their original types.
The great sub-kingdom of the Mollusca, including the bivalve and univalve shell-fishes, makes its first appearance in the Cambrian, where its earliest representatives belong to a group, the Arm-bearers or Lamp shells (Brachiopods), held by some to be allied to worms as much as to mollusks. The oldest of all these shells are allies of the modern Lingulæ ([Fig. 54]), some of the earliest of which are shown in [Fig. 55]. The modern Lingula is protected by a delicate two-valved shell, composed, unlike that of most other mollusks, of phosphate of lime or bone earth. It lives on sand-banks, attached by its long flexible stalk, which it buries like a root in the bottom. Its food consists of microscopic organisms, drifted to its mouth by cilia placed on two arm-like processes, from which the group derives its name. In the modern world about one hundred species of Brachiopods are known, belonging to about twenty genera, some of which differ considerably from the Lingulæ. The genus Terebratula, represented at [Fig. 56], is one of the most common modern as well as fossil forms, and has the valves unequal, with a round opening in one of them for the stalk, which is attached to some hard object, and there is an internal shelly loop for supporting the arms.
| Fig. 52.—Extracrinus Briareus. Reduced. Jurassic. | Fig. 53.—Pentacrinus caput-medusæ. Reduced. Modern. |
These curious, and in the modern seas, exceptional shells, were dominant in the Palæozoic period. Upwards of three thousand fossil species are known, of which a large proportion belong to the Cambrian and Silurian, nine genera appearing in the Cambrian, and no less than fifty-two in the Silurian. The history of these creatures is very remarkable. The Lingulæ, which are the first to appear, continue unchanged and with the same phosphatic shells to the present day. Morse, who has carefully studied an American species, remarks in illustration of this, that it is exceedingly tenacious of life, bearing much change of depth, temperature, etc., without being destroyed. The genus Discina, which is nearly as old, also continues throughout geological time. The genus Orthis ([Fig. 57]), which appears at the same time with the last, becomes vastly abundant in Silurian times, but dies out altogether before the end of the Palæozoic. Rhynchonella ([Fig. 58]), which comes in a little later, near the beginning of the Siluro-Cambrian, continues to this day. Spirifer and Productus ([Figs. 59 and 60]) appear later, and die out at the close of the Palæozoic. So strange and inscrutable are the fortunes of these animals, which on the whole have lost in the battle of life, that their place in nature is vastly less important than it was. It has been suggested that if any group of creatures could throw light upon the theory of descent with modification, it would be these; but Davidson, who has perhaps studied them more thoroughly than any other naturalist, found them as silent on the subject as the sponges or the corals. In a series of papers published in the Geological Magazine, a short time before his death, he remarked as follows: