The Stalk and Muscles
The body cavity of a Brachiopod is traversed by several pairs of muscles, which are very constant in position, and whose contraction serves to open and close the shell, to move the animal upon its stalk, and to govern the movements of the arms.
The stalk is absent in Crania, and the members of this genus are attached to the rocks on which they are found by the whole surface of their ventral valve. In Lingula (Fig. [315]) the stalk is long and hollow, containing what is probably a portion of the body cavity, surrounded by muscular walls. Lingula is not a fixed form, but lives half-buried in the sand of the sea-shore (Fig. [321]). Discina, the other member of the Ecardines, has a peduncle which pierces the ventral valve and fixes the animal to its support. Amongst the Testicardines, Thecidium is also fixed to its supports by the surface of its ventral valve; the other genera, however, are provided with stalks, which, being the means of the fixation of the animals, become at the same time the fixed points upon which their very limited movements can be effected. The stalk protrudes through the notch or aperture at the posterior end of the ventral valve, and it probably belongs to the ventral side of the body. It is in Cistella, and doubtless in other genera, in close organic connexion with both valves, and it seems to consist of an unusually large development of the supporting tissue which occurs so frequently in the body of Brachiopods. The surface of the peduncle is produced into several irregularities and projections which fit into any depressions of the rock upon which the animal is fixed.
In Cistella there are four pairs of muscles, two connected with opening and closing the shell, and two with the movement of the body upon the stalk (Fig. [314]). The most considerable of these muscles are the two occlusors, which have their origin, one on each side of the middle line of the dorsal valve, and their insertion by means of a tendon into the ventral valve. In the species in question each of these muscles arises by a double head, the two muscles thus formed probably representing the anterior and posterior occlusors of other forms. The contraction of these muscles undoubtedly serves to close the shell, which is opened by a small pair of divaricators arising from the ventral valve, and inserted into a portion of the dorsal shell which is posterior to the axis of the hinge. Contraction of these muscles would thus serve to approximate the posterior edges of the valves and divaricate the anterior edges and thus to open the shell.
The adjustors are four in number, a ventral pair running from the ventral valve to be inserted into the stalk, and a corresponding dorsal pair from the dorsal valve. The simultaneous contraction of either pair would tend to raise the valve, whilst the alternate contraction of the muscles of each side would tend to rotate the shell upon the peduncle. The muscles of Waldheimia flavescens are shown in Fig. [329], and described briefly on p. [502].
The muscles of the Ecardines differ from those of the Testicardines inasmuch as they do not terminate in a tendon, but the muscle fibres run straight from shell to shell. They are also more numerous. In Crania there is an anterior and a posterior pair of occlusor muscles, and two pairs of oblique muscles, which seem when they contract together to move the dorsal shell forwards, or when they contract alternately to slightly rotate it. In this genus there are also a pair of protractors and a pair of retractors, and two levators of the arms, whose function is to draw forward or retract the arms, and an unpaired median or levator ani muscle. In addition to these bundles of muscles there are certain muscles in the body wall, and it seems probable that by their contraction, when the adductors are relaxed, the body may become somewhat thicker and the valves of the shells will slightly open.
Fig. 316.—A semi-diagrammatic figure of the muscular system of Crania (after Blochmann): a, anterior occlusor; b, posterior occlusor; c, superior oblique; d, inferior oblique; e, retractor of the arms; f, elevator of the arms; g, protractor of the arms; h, unpaired median muscle. The dorsal valve is uppermost.
In Lingula (Fig. [322]) the muscular system is more complicated; in addition to the anterior (= anterior laterals) and posterior (= centrals) pairs of occlusors, there is a single divaricator (= umbonal), whose contractions in conjunction with those of certain muscles in the body wall press forward the fluid in the body cavity, and thus force the valves of the shell apart; and there are three pairs of adjustor muscles. These latter are called respectively the central (= middle laterals), external (= external laterals), and posterior (= transmedians) adjustors, whose action adjusts the shells when all contract together, and brings about a certain sliding movement of the shells on one another when they act independently of each other.[423]