Brachiopoda

BRACHIOPODA, an isolated class of marine invertebrate animals, superficially resembling bivalve molluscs. They are en closed in a shell composed of two valves which are dorso-ventrally arranged, and are equilateral and unequal in size. Two coiled appendages or brachia are developed, one on each side of the mouth and serve for the passage of food-bearing water-currents. Reproduction is sexual and the sexes are separate.

The name Brachiopod, derived from the Greek, j3paxicov =arm, robs= foot, was first proposed for the class by Dumeril (18o6) who assumed that the brachia or spiral appendages were locomo tive organs, homologous with the foot of molluscs.

Morphology.

The two valves of a Brachiopod shell are situ ated on the front (ventral valve) and back (dorsal valve) of the animal, the ventral valve being as a rule the larger. This dorso ventral arrangement of the shell distinguishes the Brachiopoda from the group of Mollusca known as Lamellibranchia (q.v.), or bivalves, which also have a shell composed of two valves, but these are situated on the sides of the animal, and are usually equal in size and unsymmetrical. In the more highly organized Brachiopods (Articulata) the two valves are united by means of teeth in the interior of the ventral valve which fit into sockets in the dorsal valve; the line of junction is known as the hinge. The hinge forms the hinder or posterior portion of the shell, and the two valves open at the opposite or anterior end. Either or both valves of the shell may be convex, but usually the ventral valve is convex and the dorsal valve flat or concave. The shell is remarkably diverse in its outline, which may be circular, oval, triangular or quadrate. Discinisca (P1. I. fig. 17) has a conical dorsal valve which is curi ously limpet-like, whereas the ventral valve of Proboscidella forms a long tube (Pl. I. fig. 1o) to which the dorsal valve acts as a lid. Another bizarre form, Pygope (P1. I. fig. 14), has a hole pierc ing the centre of the shell, dividing it into two halves which unite at the anterior margin. Generally, the shell is of small size, the largest recent species, Magellania venosa, only attaining a length of 3 +in., but many species do not exceed *in. Some fossil forms, however, were of comparatively gigantic size, Productus giganteus attaining a width of i 2in. by a length of about 9in. Many of the recent Brachiopod shells are grey or horn-coloured, while the deep sea forms have white or colourless shells which are very thin and fragile. A few genera such as Kraussina, Laqueus and Magellania are brilliantly coloured. Reddish-brown or orange pre dominates and either covers the whole shell or is distributed in patches or bands, while the brilliant greens of some species of Lingula render them objects of extreme beauty. Traces of colour may occasionally be found in fossil species such as Dielasma elon gata from the carboniferous limestone.

The shell is very diversely folded and is ornamented either by radial lines or ribs, or by transverse bands frequently bearing spines or tubercles. Primitive forms have smooth shells and many In addition to hinge-teeth and sockets the two valves in highly organized genera are held together by two sets of strong muscles, which by their contraction close or open the shell (figs. 4, 5). These muscles make scars at the point of attachment to the in terior of the valves, and these are readily seen in fossil shells. The opening muscles, or diductors, are attached to a strong median knob or cardinal process which projects slightly above the hinge of the dorsal valve, and they then stretch across to the posterior end of the ventral valve where they have a double attachment. The adductors, or closing muscles, are divided into four at the point of attachment to the dorsal valve, but are only doubly attached to the ventral valve, the two scars lying between those of the diductors. In primitive genera, such as Lingula (figs. 7, 8) in which no hinge-teeth or sockets are developed, the two valves are held together by a complex system of muscles, known as transmedians, centrals and laterals, by means of which the valves can not only open and close, but rotate and slide on one another. The shell in Lingula is composed of alternating layers of chitin and calcium phosphate covered by a smooth epidermis, while in Discinisca it is built up of overlapping lamellae of the same sub stances set at a small angle to the surface. In the hinged forms the shell is composed of three layers, (a) an outer horny epi dermis which is usually missing in fossil forms, (b) a middle lamellar layer, and (c) an inner prismatic layer. The two inner layers which are composed of calcium carbonate in the form of calcite, are frequently pierced by small canals visible at the sur face of the lamellar layer as pores. These canals contain caeca or prolongations of the two membraneous expansions, or folds of the body wall, known as the mantle. The mantle forms a lining to the shell, which it secretes, and it also acts as the principal re spiratory organ. Growth of the shell takes place by means of suc cessive additions to the margin, and thickening subsequently occurs by the deposition of new layers over the inner surface of previously formed layers.

The interior of the shell of a living Brachiopod is almost com pletely divided into two parts transversely by a membrane. The anterior part, called the mantle cavity, is occupied by the brachia or lophophore, and the posterior part, the visceral cavity or coelom, contains the alimentary canal and genital organs. The brachia consist of two spirally coiled appendages, or simply of species tend to lose their ornament and to revert to this condition in later growth-stages. The two valves are produced into a beak or umbo, that of the ventral valve being more prominent and fre quently perforated by a triangular opening or delthyrium for the passage of the pedicle, or fleshy stalk, by means of which the shell was attached to foreign objects. In primitive forms, such as Lingula, the pedicle emerges freely between the two valves, but in more advanced species the passage gradually becomes restricted to the ventral, or pedicle valve, which is either grooved or notched at the posterior end, or the opening is enclosed within the shell as in Discinisca. A calcareous plate, the deltidium, was secreted by the pedicle of some fossil forms, and gradually closed the delthyrium. This prevented the pedicle from functioning and the shells dropped from the foreign objects to which they were attached. In Produc tus there was as a rule no pedicle, and the shell was apparently supported on the muddy sea-floor by the numerous spines which ornamented the shell. Two calcareous plates, deltidial plates, are secreted on either side of the delthyrium of some hinged forms, and gradually restrict the passage of the pedicle to a rounded open ing, known as the foramen (fig. 6). The pedicle is composed of muscular fibres and is attached by strong muscles to the interior of the ventral valve just within the beak, and may be long and flexible or short and massive. In some genera the whole of the lower valve is attached and is shaped by the supporting surface, while in others such as Strophalosia the posterior part of the pedicle valve is attached in early growth stages, but later becomes free, a scar marking the former region of attachment.

two flattened disks. In hinged shells the brachia are supported by brachidia consisting either of short calcareous processes, or crura, given off below the hinge of the dorsal valve, or by loops or spiralia also supported by crura (Pl. I. fig. 6.), and often attached to a median vertical plate or septum. The brachia bear cirri or tentacles (figs. 9, io) which are fringed with numerous cilia or hairlike processes. By the constant lashing of the cilia, currents are set up within the shell by means of which food particles, such as diatoms or minute algae, are carried along a groove in the brachia to the mouth, which is medianly situated. Chitinous setae, or bristles, also fringe the edge of the mantle, and by their constant lashing cause two food-bearing currents to enter at the sides of the shell, and one current bearing waste products to leave at the anterior end. In Lingula, however, three distinct tubes are formed by these marginal setae. The mouth opens backwards into an oesophagus leading into the stomach. In primitive forms the in testine is long and terminates in an anus, but in hinged forms it is short and ends blindly, the waste products being excreted from the mouth. The liver is frequently large and many lobed, and a large part of the digestive processes is carried on in this organ.

The various internal organs are supported by bands known as mesenteries, and these further subdivide the visceral cavity, and from them spring the genital products. Extensions of the visceral cavity penetrate into the mantle and are known as the pallial or vascular sinuses. These are in the form of great trunks which branch repeatedly, and there are usually two in each mantle lobe. All contain the coelomic fluid, or blood which also fills the coelom. Circulation in Lingula is said by Morse to be entirely due to the action of cilia, while Blochmann pointed out the existence of a true heart in many genera. The organs formerly thought to be hearts are now proved to be connected with the reproductive or digestive systems. The nervous system, also not fully understood, consists of a ring round the oesophagus, and this sends out nerves to the mantle, brachia and other parts of the body. Sense organs and pigment spots are usually found in the larvae, but rarely persist in the adult. It has been claimed that the caeca of perforate genera serve in this capacity, and both Lingula and Glottidia are apparently sensitive to light.

Classification.

The class Brachiopoda has long been divided into two sub-classes based on the presence or absence of an anal opening, and of teeth and sockets in the pedicle and dorsal valves respectively. Many names have been given to these two divisions, but those generally adopted are Inarticulata and Articulata pro posed by Huxley in 1869, although Bronn's divisions Ecardines and Testicardines and Owen's Lyopomata and Arthropomata are still employed by some authors.

Beecher (1891) divided the class into four orders :—Atremata, Neotremata, Protremata and Telotremata, founded on the nature of the pedicle opening and on the shell development. The orders Atremata and Neotremata belong to Huxley's division Inarticu lata, while the Protremata and Telotremata belong to the Artic ulata. Beecher's classification emended by Schuchert in 1913 is generally adopted.