Page:Popular Science Monthly Volume 31.djvu/769

Rh I now come to my oldest noble. It is an established fact that in the most ancient strata in which fossil remains have been found, in the system called Cambrian, in the primordial fauna, shells occur which differ but little from the living arm-foots of to-day. These older or palæozoic strata fairly swarm with arm-foots; many rocks are entirely made up of them, and the richness of their forms is inexhaustible. From these earliest ages on, the animal creation gradually assumed more definite shape as the number of individual arm-foots diminished. Doubtless the more perfectly-formed organism superseded the others in the struggle for existence. The noble who relies for the support of his position only on the age of his race-stock, must die out at last if he can not adapt himself by a further development to the demands of the new time. This the arm-foots could not do: they show no progress toward a higher stage of organization. The comparatively rare brachiopods of our seas are therefore only the scanty relics of departed glory, isolated survivals of a type that was formerly wide-spread and numerous in all the seas. But it is wonderful that a race-shape should have maintained itself quite unchanged through all the geological epochs to our own time!

At shallow places in southern seas, there creep a kind of brachiopods in the sand whose shaping is rather like that of a worm than of a brachiopod. It is called the Lingula, or tongue-mussel (Fig. 3.). This creature has an even, somewhat horny shell, and a relatively long and thick stem which is clothed within by a tough layer of muscle. Along with the other species of its kind, it is distinguished from the rest of the family by the shell having no closure, while the bowel, after many turns within the body, opens without. As the valves of the shell only cover the animal, but are not closed tight, they can be easily opened and also moved sidewise upon one another. The animal is never fixed. This is the oldest animal form of the present existing creation. Lingula-shells appear in the Cambrian strata, and have been found in all the geological systems. So far as it is possible to judge from the shells, the genus has propagated itself unchanged through all the earth-history of organisms, has survived all revolutions, and has only varied into a few species differing but little from one another.

In the Silurian strata immediately following these, in which so far nearly two thousand species of arm-foots have been found, are two other still living genera: the one, Discinisca, without closure, but