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 Anodontacygnaea, 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