The race of the brachiopods goes back to the beginning of the geologic record. A few living examples are still found in the ocean, some of which, as lingula, have changed so little that they can hardly be told from the most ancient fossils of their family. Certain species are dredged abundantly on both coasts of the Atlantic from water a few fathoms deep where the bottom is rocky. They look like small mussels at first sight, but on examination show a vast difference in structure. The bivalve shells, instead of growing on the right and left sides of the animal, as in bivalve mollusks, cover its back and front, and the head parts are at the gape of the valves. At the hinge end of the shell the lower valve overlaps (it is the shape of this lower shell, like that of an old Roman lamp, which suggests their common name, "lamp shells") and the hinder end of the body projects as a stalk, by which the animal fastens itself to the rock. "The mouth in the brachiopods is flanked by two curiously coiled and feathered arms which lie within the cavity between the shells, and are supported by skeletal rods attached to the upper shell. These serve as gills, and also to capture the minute creatures upon which the brachiopod feeds."
Owing to their great abundance, world-wide distribution, and remote antiquity, as well as their excellent state of preservation, brachiopods occupy a very conspicuous rank among extinct invertebrates, and furnish us besides with a large number of important index fossils. They are to be found in immense variety from the Cambrian to the present, most numerously in formations from Silurian to Permian times.
STARFISHES, SEA URCHINS, AND TREPANGS
We have now arrived at the point (phylum Echinodermata, "spiny-skinned") where a distinctly new type of interior structure appears in the possession by animals of a hollow space (cœlom) between the outer skin and the wall of the digestive tube which now becomes occupied by definite organs instead of by an almost uniform mesenchyme, as in the sponges and cœlenterates. These organs arise from an interior lining membrane called "mesoderm."
| Photo, American Museum of Natural History |
| STARFISH AND OTHER TYPICAL LIFE IN A TIDE POOL |
Henceforth, therefore, we shall deal with cœlomate animals, among which the echinoderms are lowest in rank. The simplest of them is the "sea lily" which lives rooted on the bottom in deep water, and sways about on a slender, jointed stalk, looking much like the flower after which it is named. It is of interest chiefly as a survivor of the tribe of crinoids that were so varied and numerous in early Paleozic times that massive Devonian limestones are composed largely of their remains; and the type has changed little through the ages. It consists typically of a cup, mounted on its stem like the calyx of a flower, and composed of circles of calcareous plates, definite in form and in relative position, that contain and protect a well-organized body. Surrounding the open mouth of the cup is a circle of long, jointed, much-branched tentacles that sweep the water, capture passing prey, and bring it into the mouth of the crinoid within the circling base of the arms.
