Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/371

 BRACHIOPODA 331 spaces are as follows:—(i.) the great arm-sinus; (ii.) body-wall, and contains a portion of the coelom which, in the small arm-sinus together with the central-sinus and Discinisca and Lingula, remains in communication with the peri-oesophageal sinus, and in Discinisca and Lingula, the general body-cavity. and, to a less extent, in Crania, the lip-sinus; (iii.) certain Circulatory System.—The structures mentioned in the portions of the general body cavity which in Crania are article Brachiopoda of the ninth edition as pseudo-hearts separated off and contain muscles, &c. ; (iv.) the cavity have been shown by Huxley to be nephridia; the true of the stalk when such exists. The great arm-sinus of heart was described and figured by Hancock, but has in each side of the lophophor lies beneath the fold or lip many cases escaped the observation of later zoologists. which together with the tentacles forms the ciliated groove Blochmann in 1884, however, observed this organ in the in which the mouth opens. These sinuses are completely living animal in species of the following genera:— shut off from all other cavities, they do not open into the Terebratulina, Magellania (Waldheimia), IthynchoTiella, main ccelomic space nor into the small arm-sinus, nor does Megathyris (Argiope), Lingula, and Crania (Fig. 1). It the right sinus communicate with the left. The small consists of a definite contractile sac or sacs lying on the dorsal side of the alimentary canal near the oesophagus, and in preparations of Terebratulina made by quickly removing the viscera and examining them in sea-water under a microscope, he was able to count the pulsations, which followed one another at intervals of 30-40 seconds. A vessel'—the dorsal vessel—runs forward from the heart along the dorsal surface of the oesophagus. This vessel is nothing but a split between the right and left folds of the mesentery, and its cavity is thus a remnant of the blastocoel. A similar primitive arrangement is thought by Blochmann to obtain in the genital arteries. Anteriorly the dorsal vessel splits into a right and a left half, which enter the small arm-sinus and, running along it, give off a blind branch to each tentacle (Fig. 1). The right and left halves are connected ventrally to the oesophagus by a short vessel which supplies these tentacles in the immediate neighbourhood of the mouth. There is thus a vascular ring around the oesophagus. The heart gives off posteriorly a second median vessel which divides almost at once into a right and a left half, each of which again divides into two vessels which run to the dorsal and ventral mantles respectively. The dorsal branch sends a blind twig into each of the diverticula of the dorsal mantle-sinus, the ventral branch supplies the nephridia and neighbouring parts before reaching the ventral lobe of the mantle. Both dorsal and ventral branches supply the generative organs. The blood is a coagulable fluid. Whether it contains corpuscles is not yet determined, but if so they must be few in number. It is a remarkable fact that in Discinisca, although the vessels to the lophophor are arranged as in other Brachiopods, no trace of a heart or of the posterior vessels has as yet been discovered. The nervous system of Brachiopods has, as a rule, maintained its primitive connexion with the external epithelium. In a few places it has sunk into the connective-tissue supporting-layer beneath the ectoderm, but the chief centres still remain in the ectoderm, and the fibrils forming the nerves are for the most part at the base of the ectodermal cells. Above the oesophagus is a thin commissure which passes laterally into the chief arm-nerve. This latter includes in its course numerous ganglion cells, and forms, according to Blochmann, the immensely long drawn out supra-oesophageal ganglion. The chief arm - nerve traverses the lophophor, being situated between the great arm-sinus and the base of the lip (Figs. 2 and 3); it gives off a branch to each tentacle, and these all anastomose at the base of the tentacles with the second nerve of the arm, the so-called secondary armnerve. Like the chief arm-nerve, this strand runs through the lophophor, parallel indeed with the former except near the middle line, where it passes ventrally to the oesophagus. The lophophor is supplied by yet a third nerve, the under arm-nerve, which is less clearly defined than the others, and resembles a moderate aggregation of the nerve fibrils, which seem everywhere to underlie the ectoderm, and which in a few cases are gathered up into