The shell of a Brachiopod is secreted partly by the general surface of the body which is situated at the hinder end of the shell, and partly by the two leaf-like extensions of the body, which are termed the dorsal and ventral mantles. These are, in fact, folds of the body wall, and into them the body cavity and certain of its contents, such as the liver and generative glands, etc., extend. The space between the two folds of the mantle, which is limited behind by the anterior wall of the body, is termed the pallial or mantle cavity. On each side of the middle line the anterior wall of the body is produced into two “arms,” which occupy as a rule a considerable part of the mantle cavity. These arms may be but flattened portions of the general body wall, which occupies a large part of what in other genera is the mantle of the dorsal valve, as in Cistella and Argiope;[417] or they may be outgrowths of the body wall in the form of long processes, which are coiled and twisted in a very characteristic manner in the various genera. In any case the cross section of the arm shows a groove, one side of which forms a continuous lip, and the other takes the form of a single row of tentacles, which are richly ciliated and capable of considerable movement. The whole arm in Rhynchonella can be protruded from the shell, as was noted years ago by O. F. Müller, and although his statement to this effect has often been doubted, its truth was confirmed by Professor Morse,[418] who writes: “In the year 1872, while studying living Rhynchonella in the St. Lawrence, I observed a specimen protrude its arms to a distance of 4 c.m. beyond the anterior borders of the shell, a distance nearly equalling twice the length of the shell.” The same observer also mentions that Lingula has the power of partially protruding its arms. In most genera the cirrhi or tentacles can alone be protruded.

Fig. 314.—View of the left half of Cistella (Argiope) neapolitana, which has been cut in two by a median longitudinal incision, to show the disposition of the organs. Partly diagrammatic. The inorganic part of the shell only is shown. The tubular extensions of the mantle and the organic outer layer are not indicated, and hence the pores appear open.

The cilia which clothe the tentacles keep up a constant flow of water into the mantle cavity. This stream not only serves to aerate the blood of the animals—a process which probably takes place through the thin inner lining of the mantle—but it also brings with it a number of diatoms and other minute organisms which serve as food. These particles become entangled in the tentacles, and are ultimately lodged in the groove at their base, and passing along this by the action of the cilia they find their way into the wide mouth, into which the groove deepens in the posterior median line.

The Digestive System

The mouth leads into an oesophagus; this widens into a chamber which may be termed the stomach (Fig. [314]), and which receives the openings of two large branching glands usually known as the liver. The stomach passes into a short intestine which is usually bent at about a right angle with the oesophagus. In the Testicardines the intestine ends blindly, but in the Ecardines it is of much greater length, and terminates in an anus, situated posteriorly in the median line in Crania, but asymmetrical and to the right of the body in Lingula (Fig. [315]) and Discina; in both cases, however, the opening is into a portion of the mantle cavity. The alimentary canal is supported by a median dorsal and ventral mesentery, and by two pairs of lateral mesenteries which pass to the body wall. The lateral mesenteries are not always quite distinct. When they are, the anterior pair are known as the gastro-parietal bands, and the posterior as the ileo-parietal. In Rhynchonella there are two pairs of renal organs, and each of these mesenteries bears the internal openings of one pair. In all other Brachiopods there is only one pair, and they are supported by the ileo-parietal bands.

The alimentary canal is ciliated throughout, and some interesting observations have been made by Schulgin[419] on the shortening of these cilia in Argiope (Cistella) when the animal is well fed, and their elongation when the animal is hungry. Amongst the ciliated cells certain glandular cells have been described. The so-called liver consists of two more or less branching glands, which open by wide apertures, one on each side of the stomach. It seems probable that a good deal of digestion is carried on in these glands, since the diatoms and other minute organisms upon which the Brachiopoda live are usually found in the branches of these glands, and the glandular cells lining the tubules vary much in appearance according to the animal’s state of nutrition.

The Body Cavity

The alimentary canal and liver occupy a considerable portion of the body cavity or general space of the body; this space is to some extent cut up by the various mesenteries above mentioned. It also lodges the reproductive organs and the excretory ducts. Its walls are ciliated, and the action of the cilia keeps in motion the corpusculated fluid that bathes the various organs in the body cavity. The mantles, which are nothing but flattened leaf-like extensions of the body wall lining the shell, also contain diverticula of the body cavity, which may be simple flattered spaces or may be broken up into definite channels, as in Lingula (Fig. [315]). It seems not improbable that the body cavity fluid is aerated through the thin inner layer of the mantle.