Fig. 12.—Transverse Section of the Outer Gill-plate of Dreissensia polymorpha. (After R. H. Peck.)
f,	Constituent gill-filaments.
ff,	Fibrous sub-epidermic tissue.
ch,	Chitonous substance of the filaments.
nch,	Cells related to the chitonous substance.
lac,	Lacunar tissue.
pig,	Pigment-cells.
bc,	Blood-corpuscles.
fe,	Frontal epithelium.
lfe′, lfe″,	Two rows of latero-frontal epithelial cells with long cilia.
lrf,	Fibrous, possibly muscular,substance of the inter-  filamentar junctions.

Fig. 13.—Transverse Sections of Gill-plates of Anodonta. (After R. H. Peck.)
A,	Outer gill-plate.
B,	Inner gill-plate.
C,	A portion of B more highly magnified.
o.l,	Outer lamella.
i.l,	Inner lamella.
v,	Blood-vessel.
f,	Constituent filaments.
lac,	Lacunar tissue.
ch,	Chitonous substance of the filament.
chr,	Chitonous rod embedded in the softer substance ch.

Fig. 14.—Gill-lamellae of Anodonta. (After R. H. Peck.)

Diagram of a block cut from the outer lamella of the outer gill-plate and seen from the interlamellar surface. f, Constituent filaments; trf, fibrous tissue of the transverse inter-filamentar junctions; v, blood-vessel ilj, Inter-lamellar junction. The series of oval holes on the back of the lamella are the water-pores which open between the filaments in irregular rows separated horizontally by the transverse inter-filamentar junctions.

Fig. 15.—Diagram of a view from the left side of the animal of Anodonta cygnaea, from which the mantle-skirt, the labial tentacles and the gill-filaments have been entirely removed so as to show the relations of the axis of the gill-plumes or ctenidia g, h. (Original.)
a,	Centro-dorsal area.
b,	Anterior adductor muscle.
c,	Posterior adductor muscle.
d,	Mouth.
e,	Anus.
f,	Foot.
g,	Free portion of the axis of left ctenidium.
h,	Axis of right ctenidium.
k,	Portion of the axis of the left ctenidium which is fused with the base of the foot, the two dotted lines indicating the origins of the two rows of gill-filaments.
m,	Line of origin of the anterior labial tentacle.
n,	Nephridial aperture.
o,	Genital aperture.
r,	Line of origin of the posterior labial tentacle.

shown diagrammatically in fig. 16, C, and more correctly in fig. 17. In this region the inner lamellae of the inner gill-plates are no longer affixed to the foot. Passing still farther back behind the foot, we find in Anodonta the condition shown in the section D, fig. 16. The axes i are now free; the outer lamellae of the outer gill-plates (er) still adhere by concrescence to the mantle-skirt, whilst the inner lamellae of the inner gill-plates meet one another and fuse by concrescence at g. In the lateral view of the animal with reflected mantle-skirt and gill-plates, the line of concrescence of the inner lamellae of the inner gill-plates is readily seen; it is marked aa in fig. 1 (5). In the same figure the free part of the inner lamella of the inner gill-plate resting on the foot is marked z, whilst the attached part—the most anterior—has been snipped with scissors so as to show the genital and nephridial apertures x and y. The concrescence, then, of the free edge of the reflected lamellae of the gill-plates of Anodon is very extensive. It is important, because such a concrescence is by no means universal, and does not occur, for example, in Mytilus or in Arca; further, because when its occurrence is once appreciated, the reduction of the gill-plates of Anodonta to the plume-type of the simplest ctenidium presents no difficulty; and, lastly, it has importance in reference to its physiological significance. The mechanical result of the concrescence of the outer lamellae to the mantle-flap, and of the inner lamellae to one another as shown in section D, fig. 16, is that the sub-pallial space is divided into two spaces by a horizontal septum. The upper space (i) communicates with the outer world