The interspace between the two folds of the calcareous loop is filled up by a strong but extensile membrane, which binds them together, and forms a protecting wall to the viscera; the space between the bifurcated processes in T. Chilensis is also similarly occupied by a strong aponeurosis. In this species the muscular stem of each arm is attached to the outer sides of the loop and the intervening membrane. They commence at the pointed processes at the origin of the loop, advance along the lower portion, turn round upon the upper one, are continued along it till they reach the transverse connecting bar, where they again advance forwards, and terminate by making a half-spiral twist in front of the mouth.

Valves of Terebratula Chilensis, showing the internal framework.

Fig. 257. Valves of Terebratula Chilensis, showing the internal framework. (After Owen).

* Transactions of the Zoological Society, vol. i.

(1346). The most obvious function attributable to the tentacular organs of the animals composing this class is connected with the procurement of food; for, being utterly deprived of prehensile instruments, without some adequate contrivance these helpless creatures, imprisoned in their testaceous covering, and fixed immoveably in one locality, would be utterly unable to obtain the nourishment necessary for their support. The provision for this purpose is found in the arms, whether they be extensible or attached to calcareous loops; for these organs, being covered by cilia, produce powerful currents in the surrounding medium, which, being directed towards the mouth as to a focus, hurry into the oral aperture whatever nutritive particles may chance to be in the vicinity. The mouth itself is a simple orifice with prominent fleshy lips (fig. 256, 6), but unprovided with any dental apparatus. The alimentary canal in Lingula is a long and convoluted tube, but without a perceptible stomachal dilatation; in Terebratala, however, there is a large oval stomach (fig. 258, a, d), into which numerous ducts derived from the hepatic follicles open by large orifices. The structure of the liver in these animals is displayed by Professor Owen in the memoir from which the annexed figures are taken; and the simplicity of its organization affords an interesting lesson to the physiologist. The hepatic organ (fig. 258, a, c) consists essentially of numerous secerning caeca (fig. 258, b), as yet easily separable from each other; over which the visceral blood-vessels ramify, and bring to the secreting sacculi the circulating fluid from which the bile is elaborated.

Digestive apparatus of Terebratula. A: a, b, d, e, f, the alimentary canal; c, the hepatic caeca. B: hepatic follicles, magnified.

Fig. 258. Digestive apparatus of Terebratula. A: a, b, d, e, f, the alimentary canal; c, the hepatic caeca. B: hepatic follicles, magnified.

(1347). The Infusoria appear to be the chief food of all the Brachio-pods, perhaps the only food of the articulated species. In the latter, siliceous cases of the Diatomacece are almost always found, and sometimes in abundance. Lingula, however, appears to be a more general feeder, its intestine frequently containing a great variety of matter.

(1348). The greatest peculiarity observable in the structure of the Brachiopoda is seen in the arrangement of the respiratory system; for these animals, instead of possessing proper branchial organs, as is the case with all other Mollusca, have the mantle itself converted into a respiratory surface, and traversed by the ramifications of large bloodvessels, which form an elaborate arborescence spreading through its texture, so that it is obviously well adapted to perform the office assigned to it, more especially as its circumference is thickly studded with vibratile cilia, disposed in such a manner that by their ceaseless movements they impel continued supplies of aerated water over the whole of this vascular membrane. The lobe of the mantle which lines the perforate valve of Terebratula Chilensis (fig. 259, c) contains four large longitudinal venous trunks or sinuses (m m), and two others of similar dimensions are seen in the opposite lobe (a.) These sinuses take their origin by innumerable radicles from a circular canal of great delicacy which encompasses the entire circumference of the mantle (d).

(1349). A heart is present in all the Brachiopoda; and when in an expanded state, it is of considerable size. In the articulated species, it is appended to the middle line of the stomach, and projects freely into the perivisceral cavity, reaching down almost to the anterior margin of the oviducts. Its walls, when expanded, though rather thin, are firm, and do not collapse. They are composed of two layers, the inner of which is distinctly muscular, the fibres running in various directions, but principally radiating from centres; the outer layer is transparent and homogeneous. The interior is devoid of columnae carnese, and perfectly smooth. When this organ is in a contracted state, its size is very much reduced, the surface is slightly wrinkled, and the walls much thickened.

(1350). This unilocular heart in Waldheimia australis receives a large blood-channel or vessel in front, which, running forward along the dorsal ridge of the stomach, within the membrane denominated mesentery, comnmnicates on each side by several minute openings with the gastric lacuna, which will be more particularly noticed hereafter. The anterior extremity of this channel passes down the dorsal surface of the oesophagus, and, dividing into two lateral trunks, opens at each side into a system of large lacunes placed around the alimentary tube. This channel is the afferent cardiac channel or branchio-systemic vein.