A circumpallial sinus uniting the terminations of the pallial sinuses is figured by Joubin in Discina; and is said by Hancock, but with some doubt, to exist in Waldheimia australis, etc. Other extensions of the coelome are the sinuses at the sides of the oesophagus, which are connected to a canal coursing along the bases of the cirri in the lophophore of Argiope, or to two canals in the lophophoral arms of Crania, etc. The coelomic epithelium is ciliated partially or wholly.
The existence of a circulatory system has been denied by most modern authorities, but the recent investigations of Blochmann prove its existence, and, to a certain extent at least, as it was described by Hancock, in a large number of Brachiopoda. It appears to communicate with the coelome, but is not, as yet, known in its finer details. The heart is usually a contractile vesicle lying in the dorsal mesentery of the stomach: it is connected to a vessel (branchio-systemic vein of Hancock) which runs forwards along the oesophagus, and is only a split in the dorsal mesentery. It appears to be connected with the circum-oesophageal lacunae, and these in their turn with a vessel which runs in the sinus beneath the cirri of the lophophore and gives off a vessel to each cirrus. Genital arteries arising from the heart are also said to be present. Variations are the presence of two hearts in Argiope decollafa, and of two large and several small dilatations at the posterior end of the branchio-systemic vein in Crania anomala. Blochmann agrees with van Bemmelen in the opinion that Hancock described as blood-spaces the nervous plexuses of the arms and the branched connective tissue cells of the arms and mantle1. The coelomic fluid contains corpuscles of various kinds in suspension.
In Lingula pyramidata there are minute re-fractile globules seen also in the shell: striated spindle-shaped bodies (spermatophores): small round granular cells or young ova: and the blood corpuscles proper, rounded or oval bodies, homogeneous and nucleated. The blood corpuscles of Argiope Koivalewskii are brown, changing to red, according to Schulgin. There are no special respiratory organs, but the cirri and sinuses of the mantle must serve to aerate the coelomic fluid.
1 Beyer describes the wall of the tract in Lingula pyramidata as consisting of three layers: (1) a layer of supporting tissue; (2) one of granular small cells; (3) one of ciliated cells. Joubin figures the epithelium in Crania as consisting of extremely narrow and long ciliated cells with basal nuclei. It is possible that Beyer's second layer corresponds to the basal nuclei or to granules in the bases of the cells.
The nephridia serve the purposes of ovi- and spermi-ducts as well as of renal organs (?). They are two in number, except in the genus Rhynchonella which has four, and open internally into the coelome, and externally into the mantle cavity. The inner aperture is more or less expanded, trumpet-shaped and plicated longitudinally; it leads into a contracted tube. The lining epithelium of the tube is usually yellow in colour and is ciliated in Argiope Kowalewskii, and probably in other Brachiopoda also. The presence of coloured granules in the cells doubtless indicates a secretory function. The internal apertures are turned towards the dorsal valve. Those of the pair of organs always present are supported by the ileo-parietal bands. The ducts run in the lateral walls of the body, and they open below the convex or posterior edge of the lophophore by separate slit-like orifices. These orifices are situated in Argiope, each at the bottom of a brood-pouch or sac, invaginated from the lateral walls of the body in which the ova develope.
Thecidium has a similar, but single and median brood-pouch. The second pair of nephridia present in Rhynchonella have their internal apertures supported by the lateral gastro-parietal bands, and their external apertures are above, i. e. dorsal or anterior to the bases of the two arms.
1 Beyer describes in L. pyramidata two oblong tubular organs, one in each of the lateral oesophageal lacunae, filled with blood-corpuscles. The supporting tissue of the body-walls and mantle contain in the Brachiopod just named channels lined by coelomic epithelium, and containing blood corpuscles.
The sexes are certainly separate in Crania among Ecardines, and in many Testicardines. Beyer, the most recent investigator of Lingula (L. pyramidata),has come, like his predecessors, to the conclusion that it is hermaphrodite; Schulgin and Shipley, who have investigated Argiope, have seen none but females. As to Ecardines the genital organs of Discina are attached in groups, some to the lower edge of the lateral gastro-parietal bands, others to the ileo-parietal bands at their origin from the stomach. In Lingula anatina and L. affinis there is a set of dorsal and of ventral glands suspended to the ileo-parietal bands. L. pyramidata however agrees with Crania and other Brachiopoda in which the glands, two dorsal and two ventral, are contained in the trunks and main branches of the pallial sinuses, but extend sometimes to a greater, sometimes to a less degree into the central portion of the coelome. They are usually attached to the outer aspect of the sinuses, more rarely to the inner, i. e. the side towards the shell as in L. pyramidata and Argiope. They consist essentially of a process of supporting substance which grows out into lamellae, very numerous in the case of the testis, excavated by cavities, and covered superficially by an epithelium.
The supporting substance is continuous with that of the wall of the sinus, the epithelium with the coelomic epithelium lining it. From the epithelium are derived both sperm-cells and ova. The striated corpuscles found in the blood of Lingula are said to be spermatophores. Impregnation probably takes place after the escape of the ova. The latter during their growth (? in L. pyramidata) are surrounded by a layer of small cells, probably abortive ova.