[36] See the excellent Manual of Mollusca, by Woodward, p. 209.
The internal structure of these animals is singularly beautiful. The inner surface of each valve is lined with a soft membranous substance, called the pallial lobe, the margin of which is set round with stiff hair-like bristles, that prevent the ingress of any foreign body likely to interfere with the play of the delicate filaments of the arms. These two soft lobes are furnished with veins, and supply the place of a breathing apparatus. The body of the animal occupies not quite a third part of the interior of its valves, and is situated at the narrow end. There are thus two distinct regions within the shell, separated from each other by a strong membrane, through the centre of which is the opening of the mouth. The smaller cavity next the hinge contains the viscera, and the outer larger one, the folded and ciliated arms. These arms form one of the most characteristic features of the brachiopods. They are two in number, and proceeding from the margin of the mouth, advance into the outer empty chamber of the shell, and return upon themselves in spiral curves and folds. They are fringed with slim, flat, narrow filaments, set along the arm like teeth along the back of a fine comb. Though called arms, these long ciliated appendages are rather enormously protruded lips. The vibratory action of the fringes causes currents to set inwards towards the mouth, which is placed at the inner end or base of the arms. To support these long convoluted arms, many of the genera are furnished with slender hoops of hard calcareous matter, which are hung from the dorsal valve, and are still found within the shells of some of the most ancient fossil brachiopods.
The little visceral cavity contains the complex groups of muscles for opening and closing the valves, a simple stomach, a large granular liver, a short intestine, two hearts, and the centre of the nervous system. Without going into the details of these various structures, the reader will see that the brachiopoda are really a highly organized tribe; and I am thus particular in the enumeration, partly that he may the better understand the mechanism of the carboniferous shells of that type, and partly that he may mark how the oldest forms of life, those that meet us on the very threshold of animated existence, were not low in organization, but possessed an anatomy as complex as it was beautiful.
Who that has ever wielded an enthusiastic hammer among the richly fossiliferous beds of the mountain limestone, does not remember with delight the hosts of delicately fluted shells that the labour of an hour could pile up before him? There was the striated productus, with its slim spines scattered over the stone. There, too, lay the spirifer with its broader plications, its toothed margin, and its deeply indented valve. Less common, and so more highly prized, was the slimly-ribbed rhynconella, with its sharp, prominent beak, or perhaps the smooth, thin terebratula, with its colour-bands not yet effaced. These were pleasant hours, and their memory must dwell gratefully among the recollections of one whose avocations immure him throughout well-nigh the livelong year amid the din and dust of town—the fumum et opes strepitumque Romæ.
Fig. 22.—Productus giganteus.
In the productus the dorsal valve is sometimes quite flat, while the ventral is prominently arched, and the shell resembles a little cup with a flat plate of the same diameter placed over it. Usually, however, both the valves are concavo-convex, or arched in the same direction like two saucers placed within each other. The exterior surface of each valve is differently ornamented in the various species. A very common style of sculpturing is by a set of fine hair-like longitudinal ribs, diverging more or less regularly from the hinge line to the outer margin. In some species these ribs are wider, and are furnished with little prominent scars. In others (as P. punctatus) a set of semicircular ridges runs round the shell, narrowing as they converge from the outer lips to the centre of the hinge line, and bearing each an irregular row of small scars or tubercules. Some of the species are very irregularly ornamented into a sort of wrinkled surface, in which the striæ seem, as it were, thrown over the valves in bundles at random.
The productus was furnished with slender hollow spines, which rose up from the surface of either valve, chiefly, however, about the hinge. In P. spinosus they were long and stout, like thin rush stalks, while in the smaller species they rather resembled stiff bristles. The use of these spines is not very well made out. As most of the producti appear to have been free, that is, without any peduncle fixing them to the sea-bottom, it has been conjectured that the spines, by sinking deep into the mud, may have served the place of a peduncle to moor the shell.
As regards size, the productus is very variable. You may gather some species in the young form, not larger than peas, while others may reward your search, having a breadth of six or eight inches (P. giganteus). But however much they may vary in dimensions, they usually remain pretty constant in their abundance, being among the most common fossils of the mountain limestone, and even of some limestones in the true Coal-measures;[37] and that must be a poor stratum indeed which cannot yield you a bagful of producti.
[37] See the table given below in [Chap. X.]