Removed from its shell and suspended so as to show the general external features of the animal.
The animal is suspended by the anterior adductor muscle, and the two folds (right and left) of the mantle have been turned back and fastened over the dorsal area. The mantle-folds are free throughout their whole extent, but they are united indirectly at the posterior end behind the foot by the attachment of the branchiae. The mantle cavity is in this way divided into two chambers, an inferior large infra-branchial chamber, and a superior small supra-branchial chamber. In the living animal the ventral edges of the mantle-folds are applied more or less closely to one another, even when the foot is protruded, and water finds its entrance to the infra-branchial chamber through the inferior siphonal notch. The edges of this notch are covered with tentacles, and hence the portion of the mantle in question is readily recognised. It is posterior, and lies immediately below the attachment of the branchiae. The supra-branchial chamber opens above the attachment of the branchiae by the superior siphonal notch, which has smooth non-tentaculate edges. A white bristle is passed into it in this specimen. The two notches in many Lamelli-branchiata are prolonged, the inferior into the inhalent, the superior into the exhalent, siphon.
The two chambers, however, in the Anodon communicate one with the other, not only indirectly through the cavities of the branchiae, but directly also along the posterior part of the base of the visceral mass, where, as may be seen in this specimen, the innermost branchial lamella has a free edge. In some Lamellibranchiata with an aborted foot this gap does not exist, and consequently the two chambers do not communicate directly.
The free edges of the mantle folds are thickened, and correspond to the collar in the Snail. Two main lips, best developed posteriorly, run along the free edge, and they inclose two somewhat smaller ridges not always discernible. The outermost ridge is prolonged round the superior siphonal notch, and unites at some distance from it on the dorsal surface with its fellow to form the dorsal ridge or raphe.
The foot projects in the middle line. It is continuous with the visceral mass, which contains coils of the digestive canal, liver (=hepato-pancreas), and generative organs, and is much dilated. The foot proper is purely muscular, and may be distinguished by its yellow tint and comparative thinness. At its anterior edge a black bristle has been passed into the mouth. A ridge or lip above the mouth, and another below it, are prolonged respectively into the right and left pairs of labial tentacles, which project like wings in this specimen. Behind these tentacles, at the sides of the visceral mass, and between it and the mantle-folds, are the branchiae or gills. There are two gills or ctenidia on each side. Each gill is composed of two lamellae, an outer and an inner. The inner lamella of the inner gill is attached most anteriorly to the side of the visceral mass above the outer tentacle. It is then free for a part of its course, but posteriorly unites with its fellow, as may be seen in this specimen.
The junction of these two inner lamellae inter se separates the infra- from the supra-branchial chamber, and is the cause in part of the posterior indirect union of the mantle-folds (see also Preparation 26). The outer lamella of the outer gill is fused in its whole extent to the mantle, and it assists where the ctenidial axis becomes free posteriorly in dividing the supra- and infra-branchial chambers, and in causing the indirect union of the mantle-folds.
The internal surface of the mantle is covered with ciliated epithelium, and the plasma-cells (p. 115) of its connective tissue contain glycogen. In many forms its ventral edges coalesce or 'concresce' leaving an aperture for the foot; or the process may be carried even further when that organ is aborted, e. g. in Asper-gillum, and then the only entrance to the mantle cavity is through the inhalent siphon. When an Anodonta is removed from its shell, there is seen (best in a fresh specimen) a reddish streak running from near the anterior to near the posterior adductor. This streak is the red-brown organ of Keber, the pericardial gland of Grobben. According to the latter, it consists of a series of caeca communicating with the pericardial cavity and lined by a continuation of its epithelium. The homologous structure in Cephalopoda forms an appendix to the branchial heart, is similarly formed, and a portion of its epithelium, that in the peripheral caeca, is excretory in structure.
The foot is in some Lamellibranchiata aborted, e. g. Oysters. In others it is very small, and its shape varies much within the limits of the class. The ventral edge has, according to Griesbach, in the Anodonta three pores of fair size, one placed anteriorly, two somewhat posteriorly. He states that water finds its way through them to the vascular lacunae, a fact disputed by other authorities. Similar pores exist in other Lamellibranchiata and Mollusca (?). Water has been supposed to find its way into the blood-system in one of three ways - through the nephridium into the pericardium, through special pori aquiferi, and through intercellular passages between the ectoderm cells or epidermis. But the great distension of the foot in Anodonta, when protruded from the shell for the purposes of locomotion, is due apparently to the action of a circular muscle surrounding the vein which conveys the blood from the foot on each side of the body to the median infra-cardiac sinus. By this means the return of the blood is prevented whilst the heart at the same time continues to drive blood into the foot.
Certain regions of the mantle appear to act as blood-reservoirs from which a store of blood may be drawn under these circumstances (Fleischmann). It may be added that in Solen (Ceratisolen) legumen, where me of the blood-corpuscles are tinged with haemoglobin, these corpuscles do not escape from the blood even when the animal is greatly irritated and consequently strongly contracted. Any direct passage of water from without into the blood, or escape of blood, must consequently be regarded as an extremely doubtful occurrence. In some specimens of An-odonta there exists a small pit on the posterior margin of the foot. This, according to Carriere, is a remnant of the byssus gland, which is so well developed in Dreissena among fresh-water Bivalves. The young Anodonta (= Glochidium) has a byssus filament, but the gland which secretes it disappears, and the true byssus gland appears later. The filament referred to is adhesive, and clings to anything which it touches.