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| Fig. 24.—Magellania [Waldheimia] flavescens. Diagram showingthe muscular system. (After Hancock.) | ||
M, Ventral, N, Dorsal valve, l, Loop. V, Mouth. | Z, Extremity of intestine, c, Divaricators. c′, Accessory divaricators. a, Adductor. | b, Ventral adjusters. b′, Peduncular muscles. b″, Dorsal adjusters. P, Peduncle. |
Such is the general arrangement of the shell muscles in the division composing the articulated Brachiopoda, making allowance for certain unimportant modifications observable in the animals composing the different families and genera thereof. Owing to the strong and tight interlocking of the valves by the means of curved teeth and sockets, many species of Brachiopoda could open their valves but slightly. In some species, such as Thecidea, the animal could raise its dorsal valve at right angles to the plane of the ventral one (fig. 4).
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| Figs. 25, 26. Lingula anatina. | |
25, Interior of ventral valve. 26, Interior of dorsal valve. g, Umbonal muscular impressions (open valves). h, Central muscles (close valves). i, Transmedial or sliding muscles. | b, Parietal band. j, k, l, Lateral muscles (j, anteriors; k, middles; l,outsiders), enabling the valves to move forward and backward on each other.
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| Fig. 27.—Lingula anatina. Diagram showing the muscular system. (After Hancock.) The letters indicate the muscles as in figs. 25 and 26. |
| A, Dorsal, B, Ventral valve. p, Peduncle. e, Heart. a, Alimentary tube. z, Anal aperture. |
In the Ecardines, of which Lingula and Discina may be quoted as examples, the myology is much more complicated. Of the shell or valvular muscles W. King makes out five pairs and an odd one, and individualizes their respective functions as follows:—Three pairs are lateral, having their members limited to the sides of the shell; one pair are transmedians, each member passing across the middle of the reverse side of the shell, while the odd muscle occupies the umbonal cavity. The central and umbonal muscles effect the direct opening and closing of the shell, the laterals enable the valves to move forward and backward on each other, and the transmedians allow the similar extremities (the rostral) of the valves to turn from each other to the right or the left on an axis subcentrically situated, that is, the medio-transverse region of the dorsal valve. It was long a matter in discussion whether the animal could displace its valves sideways when about to open its shell, but this has been actually observed by Professors K. Semper and E.S. Morse, who saw the animal perform the operation. They mention that it is never done suddenly or by jerks, as the valves are at first always pushed to one side several times and back again on each other, at the same time opening gradually in the transverse direction till they rest opposite to one another and widely apart. Those who have not seen the animal in life, or who did not believe in the possibility of the valves crossing each other with a slight obliquity, would not consent to appropriating any of its muscles to that purpose, and consequently attributed to all the lateral muscles the simple function of keeping the valves in an opposite position, or holding them adjusted. We have not only the observations of Semper and Morse, but the anatomical investigations of King, to confirm the sliding action or lateral divarication of the valves of Lingula.
In the Testicardines, where no such sliding action of the valves was necessary or possible, no muscles for such an object were required, consequently none took rise from the lateral portions of the valves as in Lingula; but in an extinct group, the Trimerellidae, which seems to be somewhat intermediate in character between the Ecardines and Testicardines, have been found certain scars, which appear to have been produced by rudimentary lateral muscles, but it is doubtful (considering the shells are furnished with teeth, though but rudely developed) whether such muscles enabled the valves, as in Lingula, to move forward and backward upon each other. Crania in life opens its valves by moving upon the straight hinge, without sliding the valve.
The nervous system of Brachiopods has, as a rule, maintained its primitive connexion with the external epithelium. In a few places it has sunk into the connective-tissue supporting layer beneath the ectoderm, but the chief centres still remain in the ectoderm, and the fibrils forming the nerves are for the most part at the base of the ectodermal cells. Above the oesophagus is a thin commissure which passes laterally into the chief arm-nerve. This latter includes in its course numerous ganglion cells, and forms, according to F. Blochmann, the immensely long drawn out supra-oesophageal ganglion. The chief arm-nerve traverses the lophophore, being situated between the great arm-sinus and the base of the lip (figs. 22 and 28); it gives off a branch to each tentacle, and these all anastomose at the base of the tentacles with the second nerve of the arm, the so-called secondary arm-nerve. Like the chief arm-nerve, this strand runs through the lophophore, parallel indeed with the former except near the middle line, where it passes ventrally to the oesophagus. The lophophore is supplied by yet a third nerve, the under arm-nerve, which is less clearly defined than the others, and resembles a moderate aggregation of the nerve fibrils, which seem everywhere to underlie the ectoderm, and which in a few cases are gathered up into nerves. The under arm-nerve, which lies between the small arm-sinus and the surface, supplies nerves to the muscles of both arm-sinuses (figs. 22 and 28). Medianly, it has its origin in the sub-oesophageal ganglion, which, like the supra-oesophageal, is drawn out laterally, though not to the same extent. In the middle line the sub-oespphageal nerve mass is small; the ganglion is in fact drawn out into two halves placed on either side of the body. From each of these sub-oesophageal ganglia numerous nerves arise. Passing from the middle line outwards they are—(i.) the median pallial nerve to the middle of the dorsal mantle; (ii.) numerous small nerves—the circum-oesophageal commissures—which pass round the oesophagus to the chief arm-nerve or supra-oesophageal ganglion; (iii.) the under arm-nerve to the lophophore and its muscles; (iv.) the lateral pallial nerve to the sides of the dorsal mantle. Laterally, the sub-oesophageal ganglia give off (v.) nerves to the ventral mantle, and finally they supply (vi.) branches to the various muscles. There is a special marginal nerve running round the edge of the mantle, but the connexion of this with the rest of the nervous system is not clear; probably it is merely another concentration of the diffused sub-ectodermal nervous fibrils.
| Fig. 28.—Diagram of nervous system of Crania; from the dorsal side. The nerves running to the dorsal parts are white, with black edges; those running to the ventral parts are solid black. Magnified. (After Blochmann.) |
| 1. Oesophagus. 2. Supra-oesophageal commisure. 3. Circum-oesophageal commisure. 4. Under arm-nerve. 5. Great arm-sinus. 6. Small arm-sinus. 7. Tentacle. 8. Lip of lophophore. 9. Infra-oesophageal commisure. 10. Chief arm-nerve. 11. Secondary arm-nerve. 12. Nerves to tentacles. 13. Sub-oesophageal ganglion. 14. Dorsal lateral nerve. 15. Sub-oesophageal portion of the secondary arm-nerve. 16. Median pallial nerve of dorsal lobe of mantle. 17. Anterior occlusor muscle. 18. Posterior occlusor muscle. 19. Obliquus superior muscle. 20. Levator brachii muscle. |
The above account applies more particularly to Crania, but in the main it is applicable to the other Inarticulata which have been investigated. In Discinisca and Lingula, however, the sub-oesophageal ganglion is not drawn out, but lies medianly; it gives off two posteriorly directed nerves to the stalk, which in Lingula unite and form a substantial nerve. Sense organs are unknown in the adult. The larval forms are provided with eye-spots, but no very specialized sense organs are found in the adult.
The histology of Brachiopods presents some peculiar and many primitive features. As a rule the cells are minute, and this has especially stood in the way of embryological research. The plexus of nerve-fibrils which underlie the ectoderm and are in places gathered up into nerves, and the great development of connective tissue, are worthy of notice. Much of the latter takes the form of hyaline supporting tissue, embedded in which are scattered cells and fibres. The lophophore and stalk are largely composed of this tissue. The ectodermal cells are large, ciliated, and amongst the ciliated cells glandular cells are scattered. The chitinous chaetae have their origin in special ectodermal pits, at the base of which is one large cell which is thought to secrete the chaeta, as in Chaetopods. These pits are not isolated, but are connected by an ectodermal ridge, which grows in at the margin of the mantle and forms a continuous band somewhat resembling the ectodermal primordium of vertebrate teeth.