Fig. 24.
Pedicellina cernua. × 27.
[‘The Cambridge Natural History.’]
The polypides are borne on contractile stalks. In the Pedicellinidæ the stalks arise from a creeping stolon. In Pedicellina cernua(*) (Fig. 24) a stolon, creeping over seaweeds, etc., gives rise to stalked cups, the movements of which are vigorous: “the polypides, when excited, dash themselves vehemently from side to side. The heads are easily knocked off, but the decapitated stalks develop fresh ones. In Ascopodaria the stalks are swollen at the base; A. fruticosa(*), from Port Phillip, Victoria, forms beautiful tree-like colonies. The Loxosomidæ do not form colonies, owing to the buds becoming detached from the parent. The species of Loxosoma are always found associated with some other animal, such as a worm or Tunicate. The tentacles of the polypide are arranged obliquely to the long axis of the body, hence the name of the family (loxos, oblique). Loxosoma phascolosomatum(*) occurs, in the form of delicate tufts, on the caudal end of the Sipunculid worm Phascolosoma. The individuals resemble pins with little white heads, and are capable of vigorous movements to and fro; occasionally a stalk coils itself up into a spiral.
BRACHIOPODA.
Fig. 1.
British Brachiopods (Terebratula and Crania).
Small Table Case A against the west wall to left of main entrance.
The Brachiopoda, though presenting a certain outward resemblance to bivalved Mollusca, are quite distinct from this group. They are all marine, and all possess a bivalve shell. They grow attached to rocks (Fig. 1), usually by a horny peduncle or stalk passing between the two valves, or through a foramen in one of the latter; or, peduncle and foramen may be absent, one of the valves adhering by its surface to the rocks; some species of Lingula live in tubes in the sand or mud. They occur at all depths, from shallow water up to 2900 fathoms, but the largest number of species live at a depth of about 350 fathoms. Though found in all seas, the localities whence they have been obtained are comparatively few in number; but specimens are usually congregated in considerable numbers, in places where they do occur. The surviving species of Brachiopods constitute only a small remnant of a group that flourished abundantly in former epochs. There are about 150 recent, and over 6000 fossil species.
The Shell.—The valves of a Brachiopod shell differ from each other in size and shape, but each valve is in itself symmetrical, i.e., similar on each side of a middle line.
The valve through which the peduncle passes is termed the peduncle or ventral valve (Fig. 2, A), the other being the brachial or dorsal valve. The peduncle valve, which is usually the larger and uppermost, contains the bulk of the viscera; in the higher genera, calcareous bars or loops (Fig. 2, B) attached to the inner surface of the brachial valve form a support for the “arms” of the animal. The inner surface of the valves presents certain markings and depressions where the muscles have been attached (Fig. 6).