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by the excurrent or superior siphonal notch of the mantle (fig. 1, d); the lower space communicates by the lower siphonal notch (e in fig. 1). The only communication between the two spaces, excepting through the trellis-work of the gill-plates, is by the slit (z in fig. 1 (5)) left by the non-concrescence of a part of the inner lamella of the inner gill-plate with the foot. A probe (g) is introduced through this slit-like passage, and it is seen to pass out by the excurrent siphonal notch. It is through this passage, or indirectly through the pores of the gill-plates, that the water introduced into the lower sub-pallial space must pass on its way to the excurrent siphonal notch. Such a subdivision of the pallial chamber, and direction of the currents set up within it do not exist in a number of Lamellibranchs which have the gill-lamellae comparatively free (Mytilus, Arca, Trigonia, &c.), and it is in these forms that there is least modification by concrescence of the primary filamentous elements of the lamellae.

Fig. 16.—Diagrams of Transverse Sections of a Lamellibranch to show the Adhesion, by Concrescence, of the Gill-Lamellae to the Mantle-flaps, to the foot and to one another. (Lankester.)
A,	Shows two conditions with free gill-axis.
B,	Condition at foremost region in Anodonta.
C,	Hind region of foot in Anodonta.
D,	Region altogether posterior to the foot in Anodonta.
a,	Visceral mass.
b,	Foot.
c,	Mantle flap.
d,	Axis of gill or ctenidium.
e,	Adaxial lamella of outer gill-plate.
er,	Reflected lamella of outer gill-plate.
f,	Adaxial lamella of inner gill-plate.
fr,	Reflected lamella of inner gill-plate.
g,	Line of concrescence of the reflected lamellae of the two inner gill-plates.
h,	Rectum.
i,	Supra-branchial space of the sub-pallial chamber.

Fig. 17.—Vertical Section through an Anodonta, about the mid-region of the Foot.
m,	Mantle-flap.
br,	Outer, b′r′, inner gill-plate—each composed of two lamellae.
f,	Foot.
v,	Ventricle of the heart.
a,	Auricle.
p, p′,	Pericardial cavity.
i,	Intestine.

In the 9th edition of this Encyclopaedia Professor (Sir) E. R. Lankester suggested that these differences of gill-structure would furnish characters of classificatory value, and this suggestion has been followed out by Dr Paul Pelseneer in the classification now generally adopted.

The alimentary canal of Anodonta is shown in fig. 1 (4). The mouth is placed between the anterior adductor and the foot; the anus opens on a median papilla overlying the posterior adductor, and discharges into the superior pallial chamber along which the excurrent stream passes. The coil of the intestine in Anodonta is similar to that of other Lamellibranchs. The rectum traverses the pericardium, and has the ventricle of the heart wrapped, as it were, around it. This is not an unusual arrangement in Lamellibranchs, and a similar disposition occurs in some Gastropoda (Haliotis). A pair of ducts (ai) lead from the first enlargement of the alimentary tract called stomach into a pair of large digestive glands, the so-called liver, the branches of which are closely packed in this region (af). The food of the Anodonta, as of other Lamellibranchs, consists of microscopic animal and vegetable organisms, brought to the mouth by the stream which sets into the sub-pallial chamber at the lower siphonal notch (e in fig. 1). Probably a straining of water from solid particles is effected by the lattice-work of the ctenidia or gill-plates.

The heart of Anodonta consists of a median ventricle embracing the rectum (fig. 18, A), and giving off an anterior and a posterior artery, and of two auricles which open into the ventricle by orifices protected by valves.

Fig. 18.—Diagrams showing the Relations of Pericardium and Nephridia in a Lamellibranch such as Anodonta.
A,	Pericardium opened dorsally so as to expose the heart and the floor of the pericardial chamber d.
B,	Heart removed and floor of the pericardium cut away on the left side so as to open the non-glandular sac of the nephridium, exposing the glandular sac b, which is also cut into so as to show the probe f.
C,	Ideal pericardium and nephridium viewed laterally.
D,	Lateral view showing the actual relation of the glandular and non-glandular sacs of the nephridium. The arrows indicate the course of fluid from the pericardium outwards.
a,	Ventricle of the heart.
b,	Auricle.
bb,	Cut remnant of the auricle.
c,	Dorsal wall of the pericardium cut and reflected.
e,	Reno-pericardial orifice.
f,	Probe introduced into the left reno-pericardial orifice.
g,	Non-glandular sac of the left nephridium.
h,	Glandular sac of the left nephridium.
i,	Pore leading from the glandular into the non-glandular sac of the left nephridium.
k,	Pore leading from the non-glandular sac to the exterior.
ac,	Anterior.
ab,	Posterior, cut remnants of the intestine and ventricle.

The blood is colourless, and has colourless amoeboid corpuscles floating in it. In Ceratisolen legumen, various species of Arca and a few other species the blood is crimson, owing to the presence of corpuscles impregnated with haemoglobin. In Anodonta the blood is driven by the ventricle through the arteries into vessel-like spaces, which soon become irregular lacunae surrounding the viscera, but in parts—e.g. the labial tentacles and walls of the gut—very fine vessels with endothelial cell-lining are found. The blood makes its way by large veins to a venous sinus which lies in the middle line below the heart, having the paired renal organs (nephridia) placed between it and that organ. Hence it passes through the vessels of the glandular walls of the nephridia right and left into the gill-lamellae, whence it returns through many openings into the widely-stretched auricles. In the filaments of the gill of Protobranchia and many Filibranchia the tubular cavity is divided by a more or less complete fibrous septum into two channels, for an afferent and efferent blood-current. The ventricle and auricles of Anodonta lie in a pericardium which is clothed with a pavement endothelium (d, fig. 18).