Sub-Class Articulata

SUB-CLASS ARTICULATA Valves articulate by means of two teeth in ventral valve fitting into sockets in dorsal valve. Intestine short and ending blindly. Order 11I. Protremata, Beecher emend. Schuchert.

Forms having well-developed cardinal areas. Shell calcareous. Pedicle opening limited to ventral valve through life, and closed by a plate called the deltidium, or the shell may be cemented by the ventral valve. Brachia not supported by a calcareous skeleton. Families—Billingsellidae, Orthidae, Rhipidomellidae, Strophomen idae, *Thecidiidae, Productidae, Richthofeniidae, Syntrophiidae, Clitambonitidae, Porambonitidae, Pentameridae, Eichwaldiidae.

Order IV. Telotremata, Beecher emend. Schuchert.

Shell calcareous. Pedicle opening shared by both valves in early growth-stages, confined to the pedicle valve in the later stages, and modified by deltidial plates. Brachia supported by calcareous crura, loops or spirals. Families—Protorhynchidae, *Rhynchonellidae, Centronellidae, Stringocephalidae, *Terebratu lidae, *Terebratellidae, Atrypidae, Cyclospiridae, Spiriferidae, Suessiidae, Uncitidae, Rhynchospiridae, Meristellidae, Coelospiri dae, Athyridae.

Those starred are recent as well as f ossil.

Reproduction and

Development.—Reproduction is sexual and the sexes are separate. The male and female reproductive glands are situated in the main trunks of the vascular sinuses and possibly extend into the visceral cavity. Though attached to the inner wall of the mantle, the reproductive cells frequently leave pitted impressions on the shell which are clearly seen in fossil forms. The ova (eggs) and sperms are discharged by means of two or four funnels which open into the mantle-cavity on each side of the mouth (see fig. I 1). Fertilization takes place either within the mantle cavity or when the ova have been discharged into the sea-water. Frequently, however, the ova develop in brood pouches, formed by a fold of the mantle, within the shell of the female. The developmental stages vary considerably in the dif ferent genera. In Terebratulina (fig. 12) the ova on leaving the parent shell develop cilia and are free-swimming before becoming segmented. After a free-swimming period of about 12 days, during which time three segments are developed, the larvae settle and become fixed by the third segment which later becomes modified to form the pedicle. The lobes of the second segment grow down wards and envelop the first segment or head, and two thin, horny, semi-circular plates are developed, forming the embryonic shell or protegulum. This is soon replaced by a punctate, calcareous shell ornamented by costae, and the protegulum is rarely preserved at the umbones of the adult shell. In Lacazella (fig. 13) the dorsal lobe of the mantle develops more rapidly than the ventral, and the inner surface of both lobes bears a shell-plate. A third plate is developed on that part of the second or thoracic segment covered by the dorsal mantle-lobe. The mantle is then inverted, the mantle plates becoming external and forming with the third plate, or in cipient deltidium, the embryonic shell, or protegulum. A similar inversion of the mantle lobes occurs in Argyrotheca (fig. 14), but no third shell-plate is developed.

In Lingula, the free-swimming stage may last for a month or six weeks, and pairs of cirri on the embryonic brachia are protruded and act as a swimming organ. The cuticular shell-plate is formed over the dorsal and ven tral mantle-lobes and is circular in outline, but a fold is developed posteriorly dividing the shell into dorsal and ventral valves. After a period these become thickened and the fold of thin cuticle serves for a time as a hinge. The ped icle is formed from the ventral mantle-fold and not from the third segment as in Terebratulina.

Zoogeographical Distribu tion.—Recent Brachiopoda have a world-wide distribution in salt water and are found at various depths in all latitudes, but are only abundant in a few areas. Numerous species occur in the Mediterranean, and in the trop ical waters off the west coast of North America, the West Indies and the south coast of Australia, but it is probable that the shal low waters round the coasts of Japan contain a larger number of Brachiopods than any other area of similar extent. About 70% of living forms are found between the shore-line and a depth of r oo fathoms, and only a few spe cies inhabit mid-ocean, although quite a number live in deep water near the continental shelf. A few genera such as Lingula and Dis cinisca are littoral forms, living between the tide-marks in trop ical waters. About 200 recent species have been described ; and these are distributed among 59 genera, only seven of which are hingeless (Inarticulate) forms.

The following species have been dredged from the coasts of the British Isles :—Pelagodiscus atlanticus, Crania anomala, Crypto pora gnomon, Hemithyris psittacea, Terebratulina retusa (caput serpentis) and var. angustata, T. septentrionalis, Gwynia capsula, Argyrotheca cistellula, Megathyris detruncata, Platidia anomi oides, Pantellaria echinata, Dallina septigera, Macandrevia cra nium.

Palaeontology.

Fossil Brachiopoda considerably outnumber the recent forms, and include a large number of genera and species entirely unrepresented at the present day. Brachiopods are first found in Lower Cambrian rocks, and these early forms consist principally of hingeless genera, although a few primitive hinged forms already existed. The more specialized hinged genera (Telo tremata) appeared a little later in Ordovician times. Brachiopods became increasingly numerous in the subsequent formations, over 3,00o species being known from the Silurian rocks alone, and re markable forms occur in the carboniferous rocks, where thick beds of limestone are often entirely composed of their shells, many of which are of large size. In Jurassic and Cretaceous times Brachio pods were still numerous, but were represented almost exclusively by genera belonging to the families Rhynchonellidae and Tere bratulidae. The group waned in Tertiary times, and several species were identical with those living at the present day.

Two genera, Lingula and Crania, have persisted, practically un changed, from Ordovician to recent times, this being due, no doubt, to their remarkable adaptability to change in environment.

The evolution and affinities of this group are still obscure.

Habits.

All recent Brachiopods with the exception of Glot tidia are attached to some foreign body or to other shells of the same species, either by the pedicle, or by cementation of the lower valve. Glottidia (fig. i 6) is capable of spasmodic movements, and it builds a protecting sheath of sand-grains cemented together by mucus round its long flexible pedicle. Lingula (fig. r 5) usually makes a vertical burrow from in length, in which it lives attached by its pedicle to the sand at the bottom. By extension of the pedicle two-thirds of the shell may project above the sur face, but this disappears instantaneously when the animal is alarmed. These burrows are found in mud-flats exposed at low water, and the sand in which they are formed is black due to decaying organic matter. Clean sand is apparently unfavourable to the growth of the animal. In southern Japan and the Philippine islands the shells of Lingula are collected, and either the pedicle or the whole animal is boiled and used for food.

Historie.—The first mention of the Brachiopoda in a published work appears to have been in 1596, when Bauhin figured a Rhyn chonella from the Lias of Wurttemburg as Pectunculus biforis compressus. Ten years later Fabio Colonna described Concha anomia diphya which he considered to be a Lamellibranch but which is now known to belong to the Brachiopod genus Pygope. At the end of the 17th century Martin Lister published figures of fossil Brachiopods and Llhwyd proposed the name Terebratula for forms which had previously been confused with Anomia. At this time Brachiopods were commonly called "Lampades," or "lamp-shells." In the i 8th century several authors figured the recent species, Crania anomala, and Terebratulina retusa. Cuvier, however, in the early 19th century was the first to suggest the separation of these and similar forms from the Lamellibranchs, for which Dumeril in 18o6 proposed the name Brachiopoda. Subse quent investigation of the internal structure and development, and increasing information about fossil forms, led to the complete separation of this group from the Mollusca, Tunicata, Annelida,