The outside of the shell of recent Brachiopods is often smooth, but many are ridged. In a recent species, Rhynchonella Döderleini from Japan, Davidson[414] has described a number of spines arranged in concentric circles on the ribbed shell. They are not so long as the spines irregularly scattered on the shell of Rh. spinosa from the Jurassic formations. Some shells are brightly coloured, as, for instance, the various species of Cistella which live on the coralline rock in the Mediterranean; these exhibit bands or rays of alternate orange and bright pink. On the other hand, the shells of Terebratula vitrea are of a slightly translucent white, and of the utmost delicacy. They are very large, so that the cavity within the valves is of much greater size than the body of the animal, but in other genera the soft parts are packed very closely, and there is but a very small mantle-cavity or space within the shell unoccupied by the body of the animal. It is, however, more common for the shells of Brachiopods to be of a dull yellowish colour, and to be somewhat massive. Most species are attached by a pedicle or stalk to some rock or stone at the bottom of the sea, but in some, as in Crania, the ventral valve becomes closely adherent to its support, so much so that it is difficult, or impossible, to remove the animal without leaving the ventral valve behind. Lingula, like Crania, one of the Ecardines, lives in sand (Fig. [321], p. 483), and does not use its long pedicle to adhere to any fixed object. . The outline of the shell varies extremely. It may be almost round or prolonged along either axis; the edges of the valves may be smooth or fluted in correspondence with the ridges and grooves of the outside of the shell.
Fig. 313.—Three specimens of Crania anomala on a stone dredged in Loch Fyne. The topmost specimen is seen in profile.
On the inner surface of the shell of the Testicardinate Brachiopoda, at the hinder end of the ventral valve, are two lateral teeth, which fit into corresponding sockets in the dorsal valve. These form a hinge, which in many cases is so arranged as to permit the shell to be opened to only a very limited extent. There are also certain plate-like processes which project into the lumen of the shell, and help to support various portions of the body; and in Terebratula, Waldheimia, etc., these form a complicated band-like loop, which increases in complexity with advancing age, and serves to support the arms. In the extinct Spirifera the internal skeleton takes the form of two spirally coiled lamellae, which almost entirely fill the cavity of the shell; the apices of the spirals point outwards (Fig. [330]). The inner surface of the shell also bears the marks of the insertion of the numerous muscles which govern its movements.
Microscopic examination of thin sections of the shell shows that it consists of small prisms or spicules of calcareous substance, whose long axis lies, roughly speaking, at right angles to the surface of the shell. These spicules are held together by an organic matrix, in which, however, no cellular elements can be detected. In sections made through a decalcified shell the position of the spicules which have been dissolved by the acid is indicated by spaces, and the matrix remains as a network of fibrils, which end on the outside in a thin cuticular layer of organic matter. In Lingula and Discina the organic matter takes a much larger share in the formation of the shell, which in these genera consists of a number of alternating layers of horny and calcareous matter. The former is described by Gratiolet as fibrillated, the fibrils being obliquely placed, whilst the latter consists of a number of small prisms set at right angles to the surface of the shell.
In many genera, as in Terebratula, Terebratella, Cistella, Waldheimia, Crania, etc., the shell is pierced by a number of small canals (Fig. [314]), which in the dried specimens form so many open pores, but in the living animal contain prolongations of the mantle or body wall which secretes the shell. They contain extensions of the layer of epithelial cells which lines and secretes the shell. The canals come to the surface and at their outer end are often slightly swollen. They are closed by the cuticular layer which is mentioned above as covering the shell externally. They are not found in the loops or other internal processes of the shell. In Crania the canals depart to some extent from the usual type; instead of running a straight course to a somewhat swollen outer end, they break up into a number of very fine branching tubules, which form a very minute meshwork near the surface of the shell. These fine branches contain protoplasmic fibrils, which have their origin in the epithelial cells which lie in the tubules.
By carefully counting the number of tubules in a given area of young and old specimens of Waldheimia cranium, van Bemmelen[415] was able to show that the spaces between the tubules did not increase with age. He therefore reasoned that the shells of Brachiopoda do not increase by intussusception, and that their increase in size must be entirely due to additions made round their free edge.
The function of the tubules has been a matter of some discussion. They have been regarded as respiratory organs, but it would seem more reasonable to suppose that they serve as organs to supply nourishment, etc., to the organic matrix of the shell.[416]
With the exception of the genus Crania, it is usual for Brachiopods to bear round the edge of their mantle rows or bundles of chitinous setae or bristles (Figs. [315] and [319]). The length and arrangement of these structures vary in the different species; they are secreted from little pits in the edge of the mantle. It seems probable that they serve to some extent as organs of defence, especially in the larva, where they make their appearance at an early stage; possibly they also serve as a filter, and prevent the entrance of foreign bodies into the shell. Their presence has been taken to indicate a certain degree of affinity between Brachiopods and Chaetopods, since setae are very characteristic of the last-named group.