may be, as in Cistella, applied flat to the inner surface of the dorsal mantle fold, but more usually they are raised free from the body like a pair of moustaches, and as they are usually far too long to lie straight in the mantle cavity, they are folded or coiled up. The brachial skeleton which in many cases supports the arms has been mentioned above.
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| Fig. 21.—A diagram of the left half of an Argiope (Megathyris), which has been bisected in the median plane. |
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1. The ventral valve. |
A transverse section through the arm (fig. 22) shows that it consists of a stout base, composed of a very hyaline connective tissue not uncommon in the tissues of the Brachiopoda, which is traversed by certain canals whose nature is considered below under the section (The Body Cavity) devoted to the coelom. Anteriorly this base supports a gurrie or gutter, the pre-oral rim of which is formed by a simple lip, but the post-oral rim is composed of a closely set row of tentacles. These may number some thousands, and they are usually bent over and tend to form a closed cylinder of the gutter. Each of these tentacles (fig. 22) is hollow, and it contains a diverticulum from the coelom, a branch of the vascular system, a nerve and some muscle-fibres. Externally on two sides and on the inner surface the tentacles are ciliated, and the cilia are continued across the gutter to the lip and even on the outer surface of the latter. These cilia pass on any diatoms and other minute organism which come within their range of action to the capacious oval mouth, which appears as a mere deepening of the gutter in the middle line. In Terebratulina, Rhynchonella, Lingula, and possibly other genera, the arms can be unrolled and protruded from the opened shell; in this case the tentacles also straighten themselves and wave about in the water.
The Body Cavity.—The various internal organs of the brachiopod body, the alimentary canal and liver, the excretory organs, the heart, numerous muscles and the reproductive organs, are enclosed in a cavity called the body cavity, and since this cavity (i.) is derived from the archicoel and is from the first surrounded by meroblast, (ii.) communicates with the exterior through the nephridia or excretory organs, and (iii.) gives rise by the proliferation of the cells which line it to the ova and spermatoza, it is of the nature of a true coelom. The coelom then is a spacious chamber surrounding the alimentary canal, and is continued dorsally and ventrally into the sinuses of the mantle (fig. 21). Some of the endothelial cells lining the coelom are ciliated, the cilia keeping the corpusculated fluid contents in movement. Others of the endothelial cells show a great tendency to form muscle fibres. Besides this main coelomic cavity there are certain other spaces which F. Blochmann regards as coelomic, but it must be remembered that his interpretation rests largely on histological grounds, and at present embryological confirmation is wanting. These spaces are as follows:—(i.) the great arm-sinus; (ii.) the small arm-sinus together with the central sinus and the peri-oesophageal sinus, and in Discinisca and Lingula, and, to a less extent, in Crania, the lip-sinus; (iii.) certain portions of the general body cavity which in Crania are separated off and contain muscles, &c.; (iv.) the cavity of the stalk when such exists. The great arm-sinus of each side of the lophophore lies beneath the fold or lip which together with the tentacles forms the ciliated groove in which the mouth opens. These sinuses are completely shut off from all other cavities, they do not open into the main coelomic space nor into the small arm-sinus, nor does the right sinus communicate with the left. The small arm-sinus runs along the arms of the lophophore at the base of the tentacles, and gives off a blind diverticulum into each of these. This diverticulum contains the blood-vessel and muscle-fibres (fig. 22). In the region of the mouth where the two halves of the small arm-sinus approach one another they open into a central sinus lying beneath the oesophagus and partly walled in by the two halves of the ventral mesentery. This sinus is continued round the oesophagus as the peri-oesophageal sinus, and thus the whole complex of the small arm-sinus has the relations of the so-called vascular system of a Sipunculid. In Crania it is completely shut off from the main coelom, but in Lingula it communicates freely with this cavity. In Discinisca and Lingula there is further a lip-sinus or hollow system of channels which traverses the supporting tissue of the edge of the mantle and contains muscle-fibres. It opens into the peri-oesophageal sinus. It is better developed and more spacious in Lingula than in Discinisca. In Crania, where only indications of the lip-sinus occur, there are two other closed spaces. The posterior occlusor muscles lie in a special closed space which Blochmann also regards as coelomic. The posterior end of the intestine is similarly surrounded by a closed coelomic space known as the peri-anal sinus in which the rectum lies freely, unsupported by mesenteries. All these spaces contain a similar coagulable fluid with sparse corpuscles, and all are lined by ciliated cells. There is further a great tendency for the endothelial cells to form muscles, and this is especially pronounced in the small arm-sinus, where a conspicuous muscle is built up. The mantle-sinuses which form the chief spaces in the mantle are diverticula of the main coelomic cavity. In Discinisca they are provided with a muscular valve placed at their point of origin. They contain the same fluid as the general coelom. The stalk is an extension of the ventral body-wall, and contains a portion of the coelom which, in Discinisca and Lingula, remains in communication with the general body cavity.
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| Fig. 22.—Diagrammatic section through an arm of the lophophore of Crania. Magnified; after Blochmann. |
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1. The lip. |
The Alimentary Canal.— The mouth, which is quite devoid of armature, leads imperceptibly into a short and dorsally directed oesophagus. The latter enlarges into a spherical stomach into which open the broad ducts of the so-called liver. The stomach then passes into an intestine, which in the Testicardines (Articulata) is short, finger-shaped and closed, and in the Ecardines (Inarticulata) is longer, turned back upon its first course, and ends in an anus. In Lingula and Discina the anus lies to the right in the mantle-cavity, but in Crania it opens medianly into a posterior extension of the same. Apart from the asymmetry of the intestine caused by the lateral position of the anus in the two genera just named, Brachiopods are bilaterally symmetrical animals.
The liver consists of a right and left half, each opening by a broad duct into the stomach. Each half consists of many lobes which may branch, and the whole takes up a considerable proportion of
