This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1338). The next class of Mollusca which presents itself for our consideration was named by Cuvier on account of the remarkable character of the organs by means of which the animals composing it procure the food destined for their support. These instruments consist of two long spiral arms placed on each side of the mouth, that in many species can be unrolled to a considerable length, and protruded to some distance in search of aliment. The above character, however, taken by itself, would scarcely warrant us in considering the creatures before us as forming a separate class of Mollusca; but when, in addition to this remarkable feature in their organization, we find that they possess a respiratory apparatus peculiar to themselves, and differ widely from all other bivalves in almost every part of their structure, we feel little hesitation in continuing to regard them as distinct, and devoting the present chapter to an investigation of their anatomy.
(1339). The Brachiopoda inhabit bivalve shells, and for the most part are suspended by a fleshy tubular pedicle, resembling that of the Cirrhopods, to various submarine bodies. Such, at least, is the case in Lingula and Terebratula; but in the third genus belonging to this class, namely Orbicula, the pedicle is wanting, the lower valve of the shell being fixed immediately to the rock whereunto the animal is attached.
1 pallium, a mantle; branchiae, gills. This name, originally proposed by M. de Blainville notwithstanding his belief that the spiral arms were the organs of respiration, has since been proved by the researches of Professor Owen to be strictly appropriate to the class.
(1340). The shells of the Brachiopoda are formed upon two plans - one having the valves articulated, the other having them unarticulated*. Those with articulated valves (Waldheimia) have the hinge generally furnished with teeth and corresponding sockets, which so lock the valves together that their movements are very limited. In the unarticulated forms (Lingula) the valves do not move upon each other, for when open no part of their margins are in contact. The two divisions of the Brachiopods thus indicated will be found to be still more marked in their internal organization.
(1341). The muscular system in the Brachiopods is very complicated and peculiar in its arrangement. Five or six pairs of muscles have been described in the Terebratulidse, all of which have relation to the movements of the valves upon each other, or to their attachment to, or movements upon, the peduncle. Thus the muscles naturally divide themselves into two groups, the valvular, and those for adjusting the shell upon its peduncle. Of the former there are three pairs, which have been denominated respectively "adductors," "cardinals," and "accessory cardinals." Of the latter there are likewise three pairs, which have been designated the "dorsal pedicle muscles," the "ventral pedicle muscles, and the "capsular muscles;" the capsular muscles are, however, generally blended into one.
(1342). On separating the testaceous valves, the body of the Brachiopod is found to be enclosed between two delicate membranes, which exactly line the shell; and to these membranes, as in the case of other mollusks, the name of mantle has by common consent been appropriated. The mantle itself is thin and semitransparent; but its margins are thickened, and fringed with delicate setae.
(1343). When the two lobes of the mantle are widely divaricated, as in Lingula (fig. 256), we perceive the prominent orifice of the mouth (b) placed deeply between them: on each side of the mouth are the two fleshy fringed arms, which in this case can be protruded to a distance out of the shell, and, as Cuvier1 supposes, may act as oars, and thus enable the animal slightly to alter the position of its body; or else, as they are most probably delicate organs of touch, they may perform the office of highly sensitive tentacula.
Fig. 256. Lingula, with the valves separated: a, b, the pallium or mantle. (After Cuvier).
* "On the Organization of the Brachiopoda," by Albany Hancock, F.R.S. (Phil. Trans. 1858.) 1 "Memoire sur 1'Animal de la Lingule".
(1344). In Terebratula psittacea the arms are enormously developed, fringed upon their outer margins, and quite free except at their origins: when completely contracted, they are disposed in six or seven spiral folds; and when unfolded, they extend beyond the shell twice its longitudinal diameter. The mechanism by which they are unfolded is described by Professor Owen* as being extremely simple and beautiful. The principal stem of each arm is hollow from one end to the other, and contains a fluid, which, being acted upon by the spirally-disposed muscles forming the parietes of the canal, is forcibly injected towards the extremity of the arm, and the organ is thus expanded and protruded outwards.
(1345). In Terebratula Chilensis, on the contrary, the movements of the arms are extremely limited, and they can no longer be protruded from the shell as in the preceding species, being connected throughout their whole length with a peculiar complex testaceous apparatus attached to the internal surface of the imperforate valve of the shell (fig. 257,b), the arrangement and uses of which are thus described in the memoir above-mentioned: - The principal part of the internal framework alluded to consists of a slender flattened calcareous loop (ff), the extremities of which are attached to the lateral elevated ridges of the hinge: the crura of the loop diverge, but again approximate each other as they advance for a greater or less distance towards the opposite margin of the valve; the loop then suddenly turns towards the imperforate valve, and is bent back upon itself for a greater or less extent in different species. The loop, besides being fixed by its origins, or crura, is commonly attached to two processes (d d) going off at right angles from the sides, or formed by a bifurcation of the extremity of a central process (c),which is continued forwards from the hinge; but it is sometimes entirely free except at its origins. The arches of the loon are so slender that, notwithstanding their calcareous nature, they possess a slight degree of elasticity, and yield a little to pressure.