Fig. 315.—View of the inner side of a valve of Lingula anatifera (after François), to show the definite arrangement of the channels in the mantle: a, position of mouth; b, position of anus.
Running along the base of each arm are two canals, a small one at the base of the tentacles, which we may term the tentacular canal, and a larger one, the canal of the lip. The former sends a prolongation into each tentacle. The latter is, according to Blochmann, a closed canal in Crania, Lingula, Rhynchonella, and others; but according to Joubin,[420] it communicates in Crania at one point with the tentacular canal. It is probably originally a part of the body cavity. Blochmann[421] states in very definite terms that in Crania neither the large canal nor the small canal communicates with the general body cavity, but he admits that in Lingula the small canal opens into that space.
The Circulatory System
The details of the discovery of the central circulatory organ of Brachiopods form a curious and instructive chapter in the history of modern morphological inquiry. Hancock, in his monograph on the group, described and figured on the dorsal surface of the alimentary canal a well-developed heart, which had been previously noticed by Huxley, who first showed that the organs which up to his time had been regarded as hearts were in reality excretory organs. In connexion with this heart Hancock described numerous arteries, distributed to various parts of the body. The observers who have written upon the anatomy of Brachiopods since Hancock’s time, in spite of the fact that they had at their disposal such refined methods of research as section cutting, which was quite unknown at the time his monograph was written, have almost all failed to find this circulatory system, and many of them have been tempted to deny its existence. Blochmann,[422] however, in the year 1885 stated that he had found the heart, and had seen it pulsating in several species of Brachiopoda which he had rapidly opened whilst alive. Joubin has also described it in large specimens of Waldheimia venosa, and recently Blochmann has published a detailed account of his work on this subject. Both these authors describe the heart as a vesicle with muscular walls, situated dorsal to the alimentary canal. From this, according to Blochmann, a vessel—the branchio-visceral of Hancock—runs forward as a triangular split in the dorsal mesentery supporting the alimentary canal. This vessel divides into two at the oesophagus, and passing through some lacunae in the walls of this tube, opens into the tentacular canal, and consequently supplies the tentacles with blood. These two canals which diverge from the median artery are connected ventrally by a vessel which runs below the oesophagus; the latter is therefore surrounded by a vascular ring. Blochmann also describes two pairs of vessels that were seen and figured by Hancock. A pair of these pass over the gastro-parietal mesenteries and into the dorsal mantle sinus, the second pair pass over the ileo-parietal mesenteries and into the ventral mantle sinus; each of these four arteries runs to one of the four generative glands, which, as is so usually the case in the animal kingdom, have thus a specially rich blood supply. If this description should prove to be correct, the vascular system of Brachiopods shows a striking resemblance to that of the closed vascular system of the unarmed Gephyrea, except that the former group has specialised genital vessels. The blood is colourless.
Joubin’s description of the vascular system of W. venosa differs in some respects from that of Blochmann. He regards the heart as collecting the lymph which it receives from numerous lacunar spaces in the walls of the alimentary canal, and distributing it through various vessels, which in the main correspond with those of Blochmann, and which run both to the “arms” and to the generative glands. The latter vessels, however, open freely into the body cavity, and the fluid which is forced out from their openings freely bathes the organs found in the body cavity. Whichever of these accounts should prove to be more closely in accordance with the facts, there is little doubt that in addition to the true blood there is a corpusculated fluid in the body cavity which is to some extent kept in motion by the ciliated cells that line its walls.
The Excretory Organs
The excretory organs (kidneys) which were at one time regarded by Cuvier and Owen as hearts, are typical nephridia—that is to say, they are tubes with glandular excretory walls which open at one end by a wide but flattened funnel-shaped opening into the body cavity, and at the other end by a circular pore to the exterior (Fig. [314]). In Rhynchonella, where there are two pairs of these tubes,—the only evidence that the group presents of any metameric repetition of parts,—the inner ends of the anterior pair are supported by the gastro-parietal mesenteries, and those of the posterior pair by the ileo-parietal mesenteries. In all other Brachiopods the posterior pair alone exists. The external opening of these nephridia is near the base of the anus; in Cistella it is at the bottom of a brood-pouch formed by the tucking in of the body wall in this neighbourhood, and in this brood-pouch the eggs develop until the larval stage is reached.
The walls of these nephridia are lined by ciliated cells, amongst which are some excretory cells, in which numerous brown and yellow concretions are to be seen; these are probably the nitrogenous excreta of the animal, and pass out of the body, being washed away by the stream of water which is constantly passing between the shells.
As in so many other animals, the nephridia act as genital ducts, and through them the ova and spermatozoa, which break off from the genital glands and fall into the body cavity, find their way to the outer world.