[1] The text of Gen. iv. 22 is partly corrupt; and it is possible
that the text used by the Septuagint did not contain Cain.
[2] Gen. x. 2, Ezek. xxvii. 13.
[3] _Genesis_, in Haupt's _Sacred Books of the Old Testament_ on iv.
19, cf. also the notes on 20-22, for Lamech's family. The
identification of Lamech with _Lamga_ is also suggested by Sayce,
_Expository Times_, vii. 367. Cf. also Cheyne, "Cainites" in _Encyc.
Biblica_.
[4] _Notes on the Hebrew Text of Genesis, in loco._
LAMEGO, a city of northern Portugal, in the district of Vizeu and
formerly included in the province of Beira; 6 m. by road S. of the river
Douro and 42 m. E. of Oporto. Pop. (1900) 9471. The nearest railway
station is Peso da Regoa, on the opposite side of the Douro and on the
Barca d'Alva-Oporto railway. Lamego is an ancient and picturesque city,
in the midst of a beautiful mountain region. Its principal buildings are
the 14th-century Gothic cathedral, Moorish citadel, Roman baths and a
church which occupies the site of a mosque, and, though intrinsically
commonplace, is celebrated in Portugal as the seat of the legendary
cortes of 1143 or 1144 (see PORTUGAL, _History_). The principal
industries are viticulture and the rearing of swine, which furnish the
so-called "Lisbon hams." Lamego was a Moorish frontier fortress of some
importance in the 9th and 10th centuries. It was captured in 1057 by
Ferdinand I. of Castile and Leon.
LAMELLIBRANCHIA (Lat. _lamella_, a small or thin plate, and Gr. [Greek:
branchia], gills), the fourth of the five classes of animals
constituting the phylum Mollusca (q.v.). The Lamellibranchia are mainly
characterized by the rudimentary condition of the head, and the
retention of the primitive bilateral symmetry, the latter feature being
accentuated by the lateral compression of the body and the development
of the shell as two bilaterally symmetrical plates or valves covering
each one side of the animal. The foot is commonly a simple cylindrical
or ploughshare-shaped organ, used for boring in sand and mud, and more
rarely presents a crawling disk similar to that of Gastropoda; in some
forms it is aborted. The paired ctenidia are very greatly developed
right and left of the elongated body, and form the most prominent organ
of the group. Their function is chiefly not respiratory but nutritive,
since it is by the currents produced by their ciliated surface that
food-particles are brought to the feebly-developed mouth and buccal
cavity.
The Lamellibranchia present as a whole a somewhat uniform structure. The
chief points in which they vary are--(1) in the structure of the
ctenidia or branchial plates; (2) in the presence of one or of two chief
muscles, the fibres of which run across the animal's body from one valve
of the shell to the other (adductors); (3) in the greater or less
elaboration of the posterior portion of the mantle-skirt so as to form a
pair of tubes, by one of which water is introduced into the sub-pallial
chamber, whilst by the other it is expelled; (4) in the perfect or
deficient symmetry of the two valves of the shell and the connected soft
parts, as compared with one another; (5) in the development of the foot
as a disk-like crawling organ (_Arca_, _Nucula_, _Pectunculus_,
_Trigonia_, _Lepton_, _Galeomma_), as a simple plough-like or
tongue-shaped organ (_Unionidae_, &c.), as a re-curved saltatory organ
(_Cardium_, &c.), as a long burrowing cylinder (_Solenidae_, &c.), or
its partial (Mytilacea) or even complete abortion (Ostraeacea).
The essential Molluscan organs are, with these exceptions, uniformly
well developed. The mantle-skirt is always long, and hides the rest of
the animal from view, its dependent margins meeting in the middle line
below the ventral surface when the animal is retracted; it is, as it
were, slit in the median line before and behind so as to form two flaps,
a right and a left; on these the right and the left calcareous valves of
the shell are borne respectively, connected by an uncalcified part of
the shell called the ligament. In many embryo Lamellibranchs a
centro-dorsal primitive shell-gland or follicle has been detected. The
mouth lies in the median line anteriorly, the anus in the median line
posteriorly.
Both ctenidia, right and left, are invariably present, the axis of each
taking origin from the side of the body as in the schematic
archi-Mollusc (see fig. 15). A pair of renal tubes opening right and
left, rather far forward on the sides of the body, are always present.
Each opens by its internal extremity into the pericardium. A pair of
genital apertures, connected by genital ducts with the paired gonads,
are found right and left near the nephridial pores, except in a few
cases where the genital duct joins that of the renal organ
(_Spondylus_). The sexes are often, but not always, distinct. No
accessory glands or copulatory organs are ever present in
Lamellibranchs. The ctenidia often act as brood-pouches.
A dorsal contractile heart, with symmetrical right and left auricles
receiving aerated blood from the ctenidia and mantle-skirt, is present,
being unequally developed only in those few forms which are inequivalve.
The typical pericardium is well developed. It, as in other Mollusca, is
not a blood-space but develops from the coelom, and it communicates with
the exterior by the pair of renal tubes. As in Cephalopoda (and possibly
other Mollusca) water can be introduced through the nephridia into this
space. The alimentary canal keeps very nearly to the median vertical
plane whilst exhibiting a number of flexures and loopings in this plane.
A pair of large glandular outgrowths, the so-called "liver" or great
digestive gland, exists as in other Molluscs. A pair of pedal otocysts,
and a pair of osphradia at the base of the gills, appear to be always
present. A typical nervous system is present (fig. 19), consisting of a
cerebro-pleural ganglion-pair, united by connectives to a pedal
ganglion-pair and a visceral ganglion-pair (parieto-splanchnic).
A pyloric caecum connected with the stomach is commonly found,
containing a tough flexible cylinder of transparent cartilaginous
appearance, called the "crystalline style" (_Mactra_). In many
Lamellibranchs a gland is found on the hinder surface of the foot in the
mid line, which secretes a substance which sets into the form of
threads--the so-called "byssus"--by means of which the animal can fix
itself. Sometimes this gland is found in the young and not in the adult
(_Anodonta_, _Unio_, _Cyclas_). In some Lamellibranchs (_Pecten_,
_Spondylus_, _Pholas_, _Mactra_, _Tellina_, _Pectunculus_, _Galeomma_,
&c.), although cephalic eyes are generally absent, special eyes are
developed on the free margin of the mantle-skirt, apparently by the
modification of tentacles commonly found there. There are no pores in
the foot or elsewhere in Lamellibranchia by which water can pass into
and out of the vascular system, as formerly asserted.
The Lamellibranchia live chiefly in the sea, some in fresh waters. A
very few have the power of swimming by opening and shutting the valves
of the shell (_Pecten_, _Lima_); most can crawl slowly or burrow
rapidly; others are, when adult, permanently fixed to stones or rocks
either by the shell or the byssus. In development some Lamellibranchia
pass through a free-swimming trochosphere stage with pre-oral ciliated
band; other fresh-water forms which carry the young in brood-pouches
formed by the ctenidia have suppressed this larval phase.
[Illustration: FIG. 1.--Diagrams of the external form and anatomy of
_Anodonta cygnea_, the Pond-Mussel; in figures 1, 3, 4, 5, 6 the animal
is seen from the left side, the centro-dorsal region uppermost. (1)
Animal removed from its shell, a probe g passed into the sub-pallial
chamber through the excurrent siphonal notch. (2) View from the ventral
surface of an Anodon with its foot expanded and issuing from between the
gaping shells. (3) The left mantle-flap reflected upwards so as to
expose the sides of the body. (4) Diagrammatic section of Anodon to show
the course of the alimentary canal. (5) The two gill-plates of the left
side reflected upwards so as to expose the fissure between foot and gill
where the probe g passes. (6) Diagram to show the positions of the
nerve-ganglia, heart and nephridia.
Letters in all the figures as follows:
a, Centro-dorsal area.
b, Margin of the left mantle-flap.
c, Margin of the right mantle-flap.
d, Excurrent siphonal notch of the mantle margin.
e, Incurrent siphonal notch of the mantle margin.
f, Foot.
g, Probe passed into the superior division of the sub-pallial chamber
through the excurrent siphonal notch, and issuing by the side of the
foot into the inferior division of the sub-pallial chamber.
h, Anterior (pallial) adductor muscle of the shells.
i, Anterior retractor muscle of the foot.
k, Protractor muscle of the foot.
l, Posterior (pedal) adductor muscle of the shells.
m, Posterior retractor muscle of the foot.
n, Anterior labial tentacle.
o, Posterior labial tentacle.
p, Base-line of origin of the reflected mantle-flap from the side of
the body.
q, Left external gill-plate.
r, Left internal gill-plate.
rr, Internal lamella of the right inner gill-plate.
rg, Right outer gill-plate.
s, Line of concrescence of the outer lamella of the left outer
gill-plate with the left mantle-flap.
t, Pallial tentacles.
u, The thickened muscular pallial margin which adheres to the shell
and forms the pallial line of the left side.
v, That of the right side.
w, The mouth.
x, Aperture of the left organ of Bojanus (nephridium) exposed by
cutting the attachment of the inner lamella of the inner gill-plate.
y, Aperture of the genital duct.
z, Fissure between the free edge of the inner lamella of the inner
gill-plate and the side of the foot, through which the probe g passes
into the upper division of the sub-pallial space.
aa, Line of concrescence of the inner lamella of the right inner
gill-plate with the inner lamella of the left inner gill-plate.
ab, ac, ad, Three pit-like depressions in the median line of the foot
supposed by some writers to be pores admitting water into the vascular
system.
ae, Left shell valve.
af, Space occupied by liver.
ag, Space occupied by gonad.
ah, Muscular substance of the foot.
ai, Duct of the liver on the wall of the stomach.
ak, Stomach.
al, Rectum traversing the ventricle of the heart.
am, Pericardium.
an, Glandular portion of the left nephridium.
ap, Ventricle of the heart.
aq, Aperture by which the left auricle joins the ventricle.
ar, Non-glandular portion of the left nephridium.
as, Anus.
at, Pore leading from the pericardium into the glandular sac of the
left nephridium.
au, Pore leading from the glandular into the non-glandular portion of
the left nephridium.
av, Internal pore leading from the non-glandular portion of the left
nephridium to the external pore x.
aw, Left cerebro-pleuro-visceral ganglion.
ax, Left pedal ganglion.
ay, Left otocyst.
az, Left olfactory ganglion (parieto-splanchnic).
bb, Floor of the pericardium separating that space from the
non-glandular portion of the nephridia.]
As an example of the organization of a Lamellibranch, we shall review
the structure of the common pond-mussel or swan mussel (_Anodonta
cygnea_), comparing it with other Lamellibranchia.
The swan-mussel has superficially a perfectly developed bilateral
symmetry. The left side of the animal is seen as when removed from its
shell in fig. 1 (1). The valves of the shell have been removed by
severing their adhesions to the muscular areae h, i, k, l, m, u. The
free edge of the left half of the mantle-skirt b is represented as a
little contracted in order to show the exactly similar free edge of
the right half of the mantle-skirt c. These edges are not attached to,
although they touch, one another; each flap (right or left) can be
freely thrown back in the way carried out in fig. 1 (3) for that of
the left side. This is not always the case with Lamellibranchs; there
is in the group a tendency for the corresponding edges of the
mantle-skirt to fuse together by concrescence, and so to form a more
or less completely closed bag, as in the Scaphopoda (_Dentalium_). In
this way the notches d, e of the hinder part of the mantle-skirt of
_Anodonta_ are in the siphonate forms converted into two separate
holes, the edges of the mantle being elsewhere fused together along
this hinder margin. Further than this, the part of the mantle-skirt
bounding the two holes is frequently drawn out so as to form a pair of
tubes which project from the shell (figs. 8, 29). In such
Lamellibranchs as the oysters, scallops and many others which have the
edges of the mantle-skirt quite free, there are numerous tentacles
upon those edges. In _Anodonta_ these pallial tentacles are confined
to a small area surrounding the inferior siphonal notch (fig. 1 [3],
t). When the edges of the mantle ventral to the inhalant orifice are
united, an anterior aperture is left for the protrusion of the foot,
and thus there are three pallial apertures altogether, and species in
this condition are called "Tripora." This is the usual condition in
the Eulamellibranchia and Septibranchia. When the pedal aperture is
small and far forward there may be a fourth aperture in the region of
the fusion behind the pedal aperture. This occurs in _Solen_, and such
forms are called "Quadrifora."
[Illustration: FIG. 2.--View of the two Valves of the Shell of
_Cytherea_ (one of the Sinupalliate Isomya), from the dorsal aspect.]
The centro-dorsal point a of the animal of _Anodonta_ (fig. 1 [1]) is
called the umbonal area; the great anterior muscular surface h is that
of the anterior adductor muscle, the posterior similar surface i is
that of the posterior adductor muscle; the long line of attachment u
is the simple "pallial muscle,"--a thickened ridge which is seen to
run parallel to the margin of the mantle-skirt in this Lamellibranch.
In siphonate forms the pallial muscle is not simple, but is indented
posteriorly by a sinus formed by the muscles which retract the
siphons.
It is the approximate equality in the size of the anterior and
posterior adductor muscles which led to the name Isomya for the group
to which _Anodonta_ belongs. The hinder adductor muscle is always
large in Lamellibranchs, but the anterior adductor may be very small
(Heteromya), or absent altogether (Monomya). The anterior adductor
muscle is in front of the mouth and alimentary tract altogether, and
must be regarded as a special and peculiar development of the median
anterior part of the mantle-flap. The posterior adductor is ventral
and anterior to the anus. The former classification based on these
differences in the adductor muscles is now abandoned, having proved to
be an unnatural one. A single family may include isomyarian,
anisomyarian and monomyarian forms, and the latter in development pass
through stages in which they resemble the first two. In fact all
Lamellibranchs begin with a condition in which there is only one
adductor, and that not the posterior but the anterior. This is called
the protomonomyarian stage. Then the posterior adductor develops, and
becomes equal to the anterior, and finally in some cases the anterior
becomes smaller or disappears. The single adductor muscle of the
Monomya is separated by a difference of fibre into two portions, but
neither of these can be regarded as possibly representing the anterior
adductor of the other Lamellibranchs. One of these portions is more
ligamentous and serves to keep the two shells constantly attached to
one another, whilst the more fleshy portion serves to close the shell
rapidly when it has been gaping.
[Illustration: FIG. 3.--Right Valve of the same Shell from the Outer
Face.]
[Illustration: FIG. 4.--Left Valve of the same Shell from the Inner
Face. (Figs. 2, 3, 4 from Owen.)]
In removing the valves of the shell from an _Anodonta_, it is
necessary not only to cut through the muscular attachments of the
body-wall to the shell but to sever also a strong elastic ligament, or
spring resembling india-rubber, joining the two shells about the
umbonal area. The shell of _Anodonta_ does not present these parts in
the most strongly marked condition, and accordingly our figures (figs.
2, 3, 4) represent the valves of the sinupalliate genus _Cytherea_.
The corresponding parts are recognizable in _Anodonta_. Referring to
the figures (2, 3) for an explanation of terms applicable to the parts
of the valve and the markings on its inner surface--corresponding to
the muscular areas already noted on the surface of the animal's
body--we must specially note here the position of that denticulated
thickening of the dorsal margin of the valve which is called the hinge
(fig. 4). By this hinge one valve is closely fitted to the other.
Below this hinge each shell becomes concave, above it each shell rises
a little to form the umbo, and it is into this ridge-like upgrowth of
each valve that the elastic ligament or spring is fixed (fig. 4). As
shown in the diagram (fig. 5) representing a transverse section of the
two valves of a Lamellibranch, the two shells form a double lever, of
which the toothed-hinge is the fulcrum. The adductor muscles placed in
the concavity of the shells act upon the long arms of the lever at a
mechanical advantage; their contraction keeps the shells shut, and
stretches the ligament or spring h. On the other hand, the ligament h
acts upon the short arm formed by the umbonal ridge of the shells;
whenever the adductors relax, the elastic substance of the ligament
contracts, and the shells gape. It is on this account that the valves
of a dead Lamellibranch always gape; the elastic ligament is no longer
counteracted by the effort of the adductors. The state of closure of
the valves of the shell is not, therefore, one of rest; when it is at
rest--that is, when there is no muscular effort--the valves of a
Lamellibranch are slightly gaping, and are closed by the action of the
adductors when the animal is disturbed. The ligament is simple in
_Anodonta_; in many Lamellibranchs it is separated into two layers, an
outer and an inner (thicker and denser). That the condition of gaping
of the shell-valves is essential to the life of the Lamellibranch
appears from the fact that food to nourish it, water to aerate its
blood, and spermatozoa to fertilize its eggs, are all introduced into
this gaping chamber by currents of water, set going by the
highly-developed ctenidia. The current of water enters into the
sub-pallial space at the spot marked e in fig. 1 (1), and, after
passing as far forward as the mouth w in fig. 1 (5), takes an outward
course and leaves the sub-pallial space by the upper notch d. These
notches are known in _Anodonta_ as the afferent and efferent siphonal
notches respectively, and correspond to the long tube-like afferent
inferior and efferent superior "siphons" formed by the mantle in many
other Lamellibranchs (fig. 8).
[Illustration: FIG. 5.--Diagram of a section of a Lamellibranch's
shells, ligament and adductor muscle. a, b, right and left valves of
the shell; c, d, the umbones or short arms of the lever; e, f, the
long arms of the lever; g, the hinge; h, the ligament; i, the adductor
muscle.]
Whilst the valves of the shell are equal in _Anodonta_ we find in many
Lamellibranchs (_Ostraea_, _Chama_, _Corbula_, &c.) one valve larger,
and the other smaller and sometimes flat, whilst the larger shell may
be fixed to rock or to stones (_Ostraea_, &c.). A further variation
consists in the development of additional shelly plates upon the
dorsal line between the two large valves (_Pholadidae_). In _Pholas
dactylus_ we find a pair of umbonal plates, a dors-umbonal plate and a
dorsal plate. It is to be remembered that the whole of the cuticular
hard product produced on the dorsal surface and on the mantle-flaps is
to be regarded as the "shell," of which a median band-like area, the
ligament, usually remains uncalcified, so as to result in the
production of two valves united by the elastic ligament. But the
shelly substance does not always in boring forms adhere to this form
after its first growth. In _Aspergillum_ the whole of the tubular
mantle area secretes a continuous shelly tube, although in the young
condition two valves were present. These are seen (fig. 7) set in the
firm substance of the adult tubular shell, which has even replaced the
ligament, so that the tube is complete. In _Teredo_ a similar tube is
formed as the animal elongates (boring in wood), the original
shell-valves not adhering to it but remaining movable and provided
with a special muscular apparatus in place of a ligament. In the shell
of Lamellibranchs three distinct layers can be distinguished: an
external chitinous, non-calcified layer, the periostracum; a middle
layer composed of calcareous prisms perpendicular to the surface, the
prismatic layer; and an internal layer composed of laminae parallel to
the surface, the nacreous layer. The last is secreted by the whole
surface of the mantle except the border, and additions to its
thickness continue to be made through life. The periostracum is
produced by the extreme edge of the mantle border, the prismatic layer
by the part of the border within the edge. These two layers,
therefore, when once formed cannot increase in thickness; as the
mantle grows in extent its border passes beyond the formed parts of
the two outer layers, and the latter are covered internally by a
deposit of nacreous matter. Special deposits of the nacreous matter
around foreign bodies form pearls, the foreign nucleus being usually
of parasitic origin (see PEARL).
[Illustration: FIG. 6.--Shell of _Aspergillum vaginiferum_. (From
Owen.)]
[Illustration: FIG. 7.--Shell of _Aspergillum vaginiferum_ to show the
original valves a, now embedded in a continuous calcification of
tubular form. (From Owen.)]
Let us now examine the organs which lie beneath the mantle-skirt of
_Anodonta_, and are bathed by the current of water which circulates
through it. This can be done by lifting up and throwing back the left
half of the mantle-skirt as is represented in fig. 1 (3). We thus
expose the plough-like foot (f), the two left labial tentacles, and
the two left gill-plates or left ctenidium. In fig. 1 (5), one of the
labial tentacles n is also thrown back to show the mouth w, and the
two left gill-plates are reflected to show the gill-plates of the
right side (rr, rq) projecting behind the foot, the inner or median
plate of each side being united by concrescence to its fellow of the
opposite side along a continuous line (aa). The left inner gill-plate
is also snipped to show the subjacent orifices of the left renal organ
x, and of the genital gland (testis or ovary) y. The foot thus exposed
in _Anodonta_ is a simple muscular tongue-like organ. It can be
protruded between the flaps of the mantle (fig. 1 [1] [2]) so as to
issue from the shell, and by its action the _Anodonta_ can slowly
crawl or burrow in soft mud or sand. Other Lamellibranchs may have a
larger foot relatively than has _Anodonta_. In _Arca_ it has a
sole-like surface. In _Arca_ too and many others it carries a
byssus-forming gland and a byssus-cementing gland. In the cockles, in
_Cardium_ and in _Trigonia_, it is capable of a sudden stroke, which
causes the animal to jump when out of the water, in the latter genus
to a height of four feet. In _Mytilus_ the foot is reduced to little
more than a tubercle carrying the apertures of these glands. In the
oyster it is absent altogether.
[Illustration: FIG. 8.--_Psammobia florida_, right side, showing
expanded foot e, and g incurrent and g´ excurrent siphons. (From
Owen.)]
The labial tentacles or palps of _Anodonta_ (n, o in fig. 1 [3], [5])
are highly vascular flat processes richly supplied with nerves. The
left anterior tentacle (seen in the figure) is joined at its base in
front of the mouth (w) to the right anterior tentacle, and similarly
the left (o) and right posterior tentacles are joined behind the
mouth. Those of _Arca_ (i, k in fig. 9) show this relation to the
mouth (a). These organs are characteristic of all Lamellibranchs; they
do not vary except in size, being sometimes drawn out to streamer-like
dimensions. Their appearance and position suggest that they are in
some way related morphologically to the gill-plates, the anterior
labial tentacle being a continuation of the outer gill-plate, and the
posterior a continuation of the inner gill-plate. There is no
embryological evidence to support this suggested connexion, and, as
will appear immediately, the history of the gill-plates in various
forms of Lamellibranchs does not directly favour it. The palps are
really derived from part of the velar area of the larva.
[Illustration: FIG. 9.--View from the ventral (pedal) aspect of the
animal of _Arca noae_, the mantle-flap and gill-filaments having been
cut away. (Lankester.)
a, Mouth.
b, Anus.
c, Free spirally turned extremity of the gill-axis or ctenidial axis
of the right side.
d, Do. of the left side.
e, f, Anterior portions of these axes fused by concrescence to the
wall of the body.
g, Anterior adductor muscle.
h, Posterior adductor.
i, Anterior labial tentacle.
k, Posterior labial tentacle.
l, Base line of the foot.
m, Sole of the foot.
n, Callosity.]
The gill-plates have a structure very different from that of the
labial tentacles, and one which in _Anodonta_ is singularly
complicated as compared with the condition presented by these organs
in some other Lamellibranchs, and with what must have been their
original condition in the ancestors of the whole series of living
Lamellibranchia. The phenomenon of "concrescence" which we have
already had to note as showing itself so importantly in regard to the
free edges of the mantle-skirt and the formation of the siphons, is
what, above all things, has complicated the structure of the
Lamellibranch ctenidium. Our present knowledge of the interesting
series of modifications through which the Lamellibranch gill-plates
have developed to their most complicated form is due to R. H. Peck, K.
Mitsukuri and W. G. Ridewood. The Molluscan ctenidium is typically a
plume-like structure, consisting of a vascular axis, on each side of
which is set a row of numerous lamelliform or filamentous processes.
These processes are hollow, and receive the venous blood from, and
return it again aerated into, the hollow axis, in which an afferent
and an efferent blood-vessel may be differentiated. In the genus
_Nucula_ (fig. 10) we have an example of a Lamellibranch retaining
this plume-like form of gill. In the Arcacea (e.g. _Arca_ and
_Pectunculus_) the lateral processes which are set on the axis of the
ctenidium are not lamellae, but are slightly flattened, very long
tubes or hollow filaments. These filaments are so fine and are set so
closely together that they appear to form a continuous membrane until
examined with a lens. The microscope shows that the neighbouring
filaments are held together by patches of cilia, called "ciliated
junctions," which interlock with one another just as two brushes may
be made to do. In fig. 11, A a portion of four filaments of a
ctenidium of the sea-mussel (_Mytilus_) is represented, having
precisely the same structure as those of _Arca_. The filaments of the
gill (ctenidium) of _Mytilus_ and _Arca_ thus form two closely set
rows which depend from the axis of the gill like two parallel plates.
Further, their structure is profoundly modified by the curious
condition of the free ends of the depending filaments. These are
actually reflected at a sharp angle--doubled on themselves in
fact--and thus form an additional row of filaments (see fig. 11 B).
Consequently, each primitive filament has a descending and an
ascending ramus, and instead of each row forming a simple plate, the
plate is double, consisting of a descending and an ascending lamella.
As the axis of the ctenidium lies by the side of the body, and is very
frequently connate with the body, as so often happens in Gastropods
also, we find it convenient to speak of the two plate-like structures
formed on each ctenidial axis as the outer and the inner gill-plate;
each of these is composed of two lamellae, an outer (the reflected)
and an adaxial in the case of the outer gill-plate, and an adaxial and
an inner (the reflected) in the case of the inner gill-plate. This is
the condition seen in _Arca_ and _Mytilus_, the so-called plates
dividing upon the slightest touch into their constituent filaments,
which are but loosely conjoined by their "ciliated junctions."
Complications follow upon this in other forms. Even in _Mytilus_ and
_Arca_ a connexion is here and there formed between the ascending and
descending rami of a filament by hollow extensible outgrowths called
"interlamellar junctions" (_il._ j in B, fig. 11). Nevertheless the
filament is a complete tube formed of chitinous substance and clothed
externally by ciliated epithelium, internally by endothelium and
lacunar tissue--a form of connective tissue--as shown in fig. 11, C.
Now let us suppose as happens in the genus _Dreissensia_--a genus not
far removed from _Mytilus_--that the ciliated inter-filamentar
junctions (fig. 12) give place to solid permanent inter-filamentar
junctions, so that the filaments are converted, as it were, into a
trellis-work. Then let us suppose that the interlamellar junctions
already noted in _Mytilus_ become very numerous, large and irregular;
by them the two trellis-works of filaments would be united so as to
leave only a sponge-like set of spaces between them. Within the
trabeculae of the sponge-work blood circulates, and between the
trabeculae the water passes, having entered by the apertures left in
the trellis-work formed by the united gill-filaments (fig. 14). The
larger the intralamellar spongy growth becomes, the more do the
original gill-filaments lose the character of blood-holding tubes, and
tend to become dense elastic rods for the simple purpose of supporting
the spongy growth. This is seen both in the section of _Dreissensia_
gill (fig. 12) and in those of _Anodonta_ (fig. 13, A, B, C). In the
drawing of _Dreissensia_ the individual filaments f, f, f are cut
across in one lamella at the horizon of an inter-filamentar junction,
in the other (lower in the figure) at a point where they are free. The
chitinous substance ch is observed to be greatly thickened as compared
with what it is in fig. 11, C, tending in fact to obliterate
altogether the lumen of the filament. And in _Anodonta_ (fig. 13, C)
this obliteration is effected. In _Anodonta_, besides being thickened,
the skeletal substance of the filament develops a specially dense,
rod-like body on each side of each filament. Although the structure of
the ctenidium is thus highly complicated in _Anodonta_, it is yet more
so in some of the siphonate genera of Lamellibranchs. The filaments
take on a secondary grouping, the surface of the lamella being thrown
into a series of half-cylindrical ridges, each consisting of ten or
twenty filaments; a filament of much greater strength and thickness
than the others may be placed between each pair of groups. In
_Anodonta_, as in many other Lamellibranchs, the ova and hatched
embryos are carried for a time in the ctenidia or gill apparatus, and
in this particular case the space between the two lamellae of the
outer gill-plate is that which serves to receive the ova (fig. 13, A).
The young are nourished by a substance formed by the cells which cover
the spongy interlamellar outgrowths.
[Illustration: FIG. 10.--Structure of the Ctenidia of _Nucula_. (After
Mitsukuri.) See also fig. 2.
A. Section across the axis of a ctenidium with a pair of
plates--flattened and shortened filaments--attached.
i, j, k, g, Are placed on or near the membrane which attaches the
axis of the ctenidium to the side of the body.
a, b, Free extremities of the plates (filaments).
d, Mid-line of the inferior border.
e, Surface of the plate.
t, Its upper border.
h, Chitinous lining of the plate.
r, Dilated blood-space.
u, Fibrous tract.
o, Upper blood-vessel of the axis.
n, Lower blood-vessel of the axis.
s, Chitinous framework of the axis.
cp, Canal in the same.
A, B, Line along which the cross-section C of the plate is taken.
B. Animal of a male _Nucula proxima_, Say, as seen when the left
valve of the shell and the left half of the mantle-skirt are
removed.
a, a, Anterior adductor muscle.
p.a, Posterior adductor muscle.
v.m, Visceral mass.
f, Foot.
g, Gill.
l, Labial Tentacle.
l.a, Filamentous appendage of the labial tentacle.
lb, Hood-like appendage of the labial tentacle.
m, Membrane suspending the gill and attached to the body along the
line x, y, z, w.
p, Posterior end of the gill (ctenidium).
C. Section across one of the gill-plates (A, B, in A) comparable
with fig. 11 C.
i.a, Outer border.
d.a, Axial border.
l.f, Latero-frontal epithelium.
e, Epithelium of general surface.
r, Dilated blood-space.
h, Chitinous lining (compare A).]
[Illustration: FIG. 11.--Filaments of the Ctenidium of _Mytilus
edulis_. (After R. H. Peck.)
A, Part of four filaments seen from the outer face in order to show
the ciliated junctions c.j.
B, Diagram of the posterior face of a single complete filament with
descending ramus and ascending ramus ending in a hook-like process;
ep., ep., the ciliated junctions; il, j., interlamellar junction.
C, Transverse section of a filament taken so as to cut neither a
ciliated junction nor an interlamellar junction. f.e., Frontal
epithelium; l.f.e´., l.f.e´´., the two rows of latero-frontal
epithelial cells with long cilia; ch, chitinous tubular lining of
the filament; lac., blood lacuna traversed by a few processes of
connective tissue cells; b.c., blood-corpuscle.]
Other points in the modification of the typical ctenidium must be
noted in order to understand the ctenidium of _Anodonta_. The axis of
each ctenidium, right and left, starts from a point well forward near
the labial tentacles, but it is at first only a ridge, and does not
project as a free cylindrical axis until the back part of the foot is
reached. This is difficult to see in _Anodonta_, but if the
mantle-skirt be entirely cleared away, and if the dependent lamellae
which spring from the ctenidial axis be carefully cropped so as to
leave the axis itself intact, we obtain the form shown in fig. 15,
where g and h are respectively the left and the right ctenidial axes
projecting freely beyond the body. In _Arca_ this can be seen with far
less trouble, for the filaments are more easily removed than are the
consolidated lamellae formed by the filaments of _Anodonta_, and in
_Arca_ the free axes of the ctenidia are large and firm in texture
(fig. 9, c, d).
[Illustration: 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.]
[Illustration: 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.]
[Illustration: 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.]
[Illustration: 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.]
If we were to make a vertical section across the long axis of a
Lamellibranch which had the axis of its ctenidium free from its origin
onwards, we should find such relations as are shown in the diagram
fig. 16, A. The gill axis d is seen lying in the sub-pallial chamber
between the foot b and the mantle c. From it depend the gill-filaments
or lamellae--formed by united filaments--drawn as black lines f. On
the left side these lamellae are represented as having only a small
reflected growth, on the right side the reflected ramus or lamella is
complete (fr and er). The actual condition in _Anodonta_ at the region
where the gills begin anteriorly is shown in fig. 16, B. The axis of
the ctenidium is seen to be adherent to, or fused by concrescence
with, the body-wall, and moreover on each side the outer lamella of
the outer gill-plate is fused to the mantle, whilst the inner lamella
of the inner gill-plate is fused to the foot. If we take another
section nearer the hinder margin of the foot, we get the arrangement
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 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.
[Illustration: 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.]
[Illustration: 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.
[Illustration: 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). It does not contain blood or communicate
directly with the blood-system; this isolation of the pericardium we
have noted already in Gastropods and Cephalopods. A good case for the
examination of the question as to whether blood enters the pericardium
of Lamellibranchs, or escapes from the foot, or by the renal organs
when the animal suddenly contracts, is furnished by the _Ceratisolen
legumen_, which has red blood-corpuscles. According to observations
made by Penrose on an uninjured _Ceratisolen legumen_, no red
corpuscles are to be seen in the pericardial space, although the heart
is filled with them, and no such corpuscles are ever discharged by the
animal when it is irritated.
[Illustration: FIG. 19.--Nerve-ganglia and Cords of three
Lamellibranchs. (From Gegenbaur.)
A, Of _Teredo_.
B, Of _Anodonta_.
C, Of _Pecten_.
a, Cerebral ganglion-pair (= cerebro-pleuro-visceral).
b, Pedal ganglion-pair.
c, Olfactory (osphradial) ganglion-pair.]
[Illustration: FIG. 20.--Otocyst of _Cyclas_. (From Gegenbaur.)
c, Capsule.
e, Ciliated cells lining the same.
o, Otolith.]
The pair of renal organs of _Anodonta_, called in Lamellibranchs the
organs of Bojanus, lie below the membranous floor of the pericardium,
and open into it by two well-marked apertures (e and f in fig. 18).
Each nephridium, after being bent upon itself as shown in fig. 18, C,
D, opens to the exterior by a pore placed at the point marked x in
fig. 1 (5) (6). One half of each nephridium is of a dark-green colour
and glandular (h in fig. 18). This opens into the reflected portion
which overlies it as shown in the diagram fig. 18, D, i; the latter
has non-glandular walls, and opens by the pore k to the exterior. The
renal organs may be more ramified in other Lamellibranchs than they
are in _Anodonta_. In some they are difficult to discover. That of the
common oyster was described by Hoek. Each nephridium in the oyster is
a pyriform sac, which communicates by a narrow canal with the
urino-genital groove placed to the front of the great adductor muscle;
by a second narrow canal it communicates with the pericardium. From
all parts of the pyriform sac narrow stalk-like tubes are given off,
ending in abundant widely-spread branching glandular caeca, which form
the essential renal secreting apparatus. The genital duct opens by a
pore into the urino-genital groove of the oyster (the same arrangement
being repeated on each side of the body) close to but distinct from
the aperture of the nephridial canal. Hence, except for the formation
of a urino-genital groove, the apertures are placed as they are in
_Anodonta_. Previously to Hoek's discovery a brown-coloured investment
of the auricles of the heart of the oyster had been supposed to
represent the nephridia in a rudimentary state. This investment, which
occurs also in many Filibranchia, forms the pericardial glands,
comparable to the pericardial accessory glandular growths of
Cephalopoda. In _Unionidae_ and several other forms the pericardial
glands are extended into diverticula of the pericardium which
penetrate the mantle and constitute the organ of Heber. The glands
secrete hippuric acid which passes from the pericardium into the renal
organs.
_Nervous System and Sense-Organs._--In _Anodonta_ there are three
well-developed pairs of nerve ganglia (fig. 19, B, and fig. 1 (6)). An
anterior pair, lying one on each side of the mouth (fig. 19, B, a) and
connected in front of it by a commissure, are the representatives of
the cerebral and pleural ganglia of the typical Mollusc, which are not
here differentiated as they are in Gastropods. A pair placed close
together in the foot (fig. 19, B, b, and fig. 1 (6), ax) are the
typical pedal ganglia; they are joined to the cerebro-pleural ganglia
by connectives.
Posteriorly beneath the posterior adductors, and covered only by a
thin layer of elongated epidermal cells, are the visceral ganglia.
United with these ganglia on the outer sides are the osphradial
ganglia, above which the epithelium is modified to form a pair of
sense-organs, corresponding to the osphradia of other Molluscs. In
some Lamellibranchs the osphradial ganglia receive nerve-fibres, not
from the visceral ganglia, but from the cerebral ganglia along the
visceral commissure. Formerly the posterior pair of ganglia were
identified as simply the osphradial ganglia, and the anterior pair as
the cerebral, pleural and visceral ganglia united into a single pair.
But it has since been discovered that in the Protobranchia the
cerebral ganglia and the pleural are distinct, each giving origin to
its own connective which runs to the pedal ganglion. The cerebro-pedal
and pleuro-pedal connectives, however, in these cases are only
separate in the initial parts of their course, and unite together for
the lower half of their length, or for nearly the whole length.
Moreover, in many forms, in which in the adult condition there is only
a single pair of anterior ganglia and a single pedal connective, a
pleural ganglion distinct from the cerebral has been recognized in the
course of development. There is, however, no evidence of the union of
a visceral pair with the cerebro-pleural.
[Illustration: FIG. 21.--Pallial Eye of _Spondylus_. (From Hickson.)
a, Prae-corneal epithelium.
b, Cellular lens.
c, Retinal body.
d, Tapetum.
e, Pigment.
f, Retinal nerve.
g, Complementary nerve.
h, Epithelial cells filled with pigment.
k, Tentacle.]
The sense-organs of _Anodonta_ other than the osphradia consist of a
pair of otocysts attached to the pedal ganglia (fig. 1 (6), ay). The
otocysts of _Cyclas_ are peculiarly favourable for study on account of
the transparency of the small foot in which they lie, and may be taken
as typical of those of Lamellibranchs generally. The structure of one
is exhibited in fig. 20. A single otolith is present as in the veliger
embryos of Opisthobranchia. In Filibranchia and many Protobranchia the
otocyst (or statocyst) contains numerous particles (otoconia). The
organs are developed as invaginations of the epidermis of the foot,
and in the majority of the Protobranchia the orifice of invagination
remains open throughout life; this is also the case in _Mytilus_
including the common mussel.
[Illustration: FIG. 22.--Two Stages in the Development of _Anodonta_.
(From Balfour.) Both figures represent the glochidium stage.
A, When free swimming, shows the two dentigerous valves widely open.
B, A later stage, after fixture to the fin of a fish.
sh, Shell.
ad, Adductor muscle.
s, Teeth of the shell.
by, Byssus.
a.ad, Anterior adductor.
p.ad, Posterior adductor.
mt, Mantle-flap.
f, Foot.
br, Branchial filaments.
au.v, Otocyst.
al, Alimentary canal.]
_Anodonta_ has no eyes of any sort, and the tentacles on the mantle
edge are limited to its posterior border. This deficiency is very
usual in the class; at the same time, many Lamellibranchs have
tentacles on the edge of the mantle supplied by a pair of large
well-developed nerves, which are given off from the cerebro-pleural
ganglion-pair, and very frequently some of these tentacles have
undergone a special metamorphosis converting them into
highly-organized eyes. Such eyes on the mantle-edge are found in
_Pecten_, _Spondylus_, _Lima_, _Pinna_, _Pectunculus_, _Modiola_,
_Cardium_, _Tellina_, _Mactra_, _Venus_, _Solen_, _Pholas_ and
_Galeomma_. They are totally distinct from the cephalic eyes of
typical Mollusca, and have a different structure and historical
development. They have originated not as pits but as tentacles. They
agree with the dorsal eyes of _Oncidium_ (Pulmonata) in the curious
fact that the optic nerve penetrates the capsule of the eye and passes
in front of the retinal body (fig. 21), so that its fibres join the
anterior faces of the nerve-end cells as in Vertebrates, instead of
their posterior faces as in the cephalic eyes of Mollusca and
Arthropoda; moreover, the lens is not a cuticular product but a
cellular structure, which, again, is a feature of agreement with the
Vertebrate eye. It must, however, be distinctly borne in mind that
there is a fundamental difference between the eye of Vertebrates and
of all other groups in the fact that in the Vertebrata the retinal
body is itself a part of the central nervous system, and not a
separate modification of the epidermis--myelonic as opposed to
epidermic. The structure of the reputed eyes of several of the
above-named genera has not been carefully examined. In _Pecten_ and
_Spondylus_, however, they have been fully studied (see fig. 21, and
explanation). Rudimentary cephalic eyes occur in the _Mytilidae_ and
in _Avicula_ at the base of the first filament of the inner gill, each
consisting of a pigmented epithelial fossa containing a cuticular
lens. In the _Arcidae_ the pallial eyes are compound or faceted
somewhat like those of Arthropods.
[Illustration: FIG. 23.--Development of the Oyster, _Ostrea edulis_.
(Modified from Horst.)
A, Blastula stage (one-cell-layered sac), with commencing
invagination of the wall of the sac at bl, the blastopore.
B, Optical section of a somewhat later stage, in which a second
invagination has begun--namely, that of the shell-gland sk.
bl, Blastopore.
en, Invaginated endoderm (wall of the future arch-enteron).
ec, Ectoderm.
C, Similar optical section at a little later stage. The
invagination connected with the blastopore is now more contracted,
d; and cells, me, forming the mesoblast from which the coelom and
muscular and skeleto-trophic tissues develop, are separated.
D, Similar section of a later stage. The blastopore, bl, has
closed; the anus will subsequently perforate the corresponding
area. A new aperture, m, the mouth, has eaten its way into the
invaginated endodermal sac, and the cells pushed in with it
constitute the stomodaeum. The shell-gland, sk, is flattened out,
and a delicate shell, s, appears on its surface. The ciliated velar
ring is cut in the section, as shown by the two projecting cilia on
the upper part of the figure. The embryo is now a Trochosphere.
E, Surface view of an embryo at a period almost identical with that
of D.
F, Later embryo seen as a transparent object.
m, Mouth.
ft, Foot.
a, Anus.
e, Intestine.
st, Stomach.
tp, Velar area of the prostomium. The extent of the shell and
commencing upgrowth of the mantle-skirt is indicated by a line
forming a curve from a to F.
_N.B._--In this development, as in that of _Pisidium_ (fig. 25), no
part of the blastopore persists either as mouth or as anus, but the
aperture closes--the pedicle of invagination, or narrow neck of the
invaginated arch-enteron, becoming the intestine. The mouth and the
anus are formed as independent in-pushings, the mouth with
stomodaeum first, and the short anal proctodaeum much later. This
interpretation of the appearances is contrary to that of Horst, from
whom our drawings of the oyster's development are taken. The account
given by the American William K. Brooks differs greatly as to matter
of fact from that of Horst, and appears to be erroneous in some
respects.]
[Illustration: FIG. 24.--Embryo of _Pisidium pusillum_ in the
diblastula stage, surface view (after Lankester). The embryo has
increased in size by accumulation of liquid between the outer and the
invaginated cells. The blastopore has closed.]
[Illustration: FIG. 25.--B, Same embryo as fig. 24, in optical median
section, showing the invaginated cells hy which form the arch-enteron,
and the mesoblastic cells me which are budded off from the surface of
the mass hy, and apply themselves to the inner surface of the
epiblastic cell-layer ep. C. The same embryo focused so as to show the
mesoblastic cells which immediately underlie the outer cell-layer.]
_Generative Organs._--The gonads of _Anodonta_ are placed in distinct
male and female individuals. In some Lamellibranchs--for instance, the
European Oyster and the _Pisidium pusillum_--the sexes are united in
the same individual; but here, as in most hermaphrodite animals, the
two sexual elements are not ripe in the same individual at the same
moment. It has been conclusively shown that the _Ostrea edulis_ does
not fertilize itself. The American Oyster (_O. virginiana_) and the
Portuguese Oyster (_O. angulata_) have the sexes separate, and
fertilization is effected in the open water after the discharge of the
ova and the spermatozoa from the females and males respectively. In
the _Ostrea edulis_ fertilization of the eggs is effected at the
moment of their escape from the uro-genital groove, or even before, by
means of spermatozoa drawn into the sub-pallial chamber by the
incurrent ciliary stream, and the embryos pass through the early
stages of development whilst entangled between the gill-lamellae of
the female parent (fig. 23). In _Anodonta_ the eggs pass into the
space between the two lamellae of the outer gill-plate, and are there
fertilized, and advance whilst still in this position to the
glochidium phase of development (fig. 22). They may be found here in
thousands in the summer and autumn months. The gonads themselves are
extremely simple arborescent glands which open to the exterior by two
simple ducts, one right and one left, continuous with the tubular
branches of the gonads. In the most primitive Lamellibranchs there is
no separate generative aperture but the gonads discharge into the
renal cavity, as in _Patella_ among Gastropods. This is the case in
the Protobranchia, e.g. _Solenomya_, in which the gonad opens into the
reno-pericardial duct. But the generative products do not pass through
the whole length of the renal tube: there is a direct opening from the
pericardial end of the tube to the distal end, and the ova or sperms
pass through this. In _Arca_, in _Anomiidae_ and in _Pectinidae_ the
gonad opens into the external part of the renal tube. The next stage
of modification is seen in _Ostraea_, _Cyclas_ and some _Lucinidae_,
in which the generative and renal ducts open into a cloacal slit on
the surface of the body. In _Mytilus_ the two apertures are on a
common papilla, in other cases the two apertures are as in _Anodonta_.
The Anatinacea and _Poromya_ among the Septibranchia are, however,
peculiar in having two genital apertures on each side, one male and
one female. These forms are hermaphrodite, with an ovary and testis
completely separate from each other on each side of the body, each
having its own duct and aperture.
The development of _Anodonta_ is remarkable for the curious larval
form known as _glochidium_ (fig. 22). The glochidium quits the
gill-pouch of its parent and swims by alternate opening and shutting
of the valves of its shell, as do adult _Pecten_ and _Lima_, trailing
at the same time a long byssus thread. This byssus is not homologous
with that of other Lamellibranchs, but originates from a single
glandular epithelial cell embedded in the tissues on the dorsal
anterior side of the adductor muscle. By this it is brought into
contact with the fin of a fish, such as perch, stickleback or others,
and effects a hold thereon by means of the toothed edge of its shells.
Here it becomes encysted, and is nourished by the exudations of the
fish. It remains in this condition for a period of two to six weeks,
and during this time the permanent organs are developed from the cells
of two symmetrical cavities behind the adductor muscle. The early
larva of _Anodonta_ is not unlike the trochosphere of other
Lamellibranchs, but the mouth is wanting. The glochidium is formed by
the precocious development of the anterior adductor and the
retardation of all the other organs except the shell. Other
Lamellibranchs exhibit either a trochosphere larva which becomes a
veliger differing only from the Gastropod's and Pteropod's veliger in
having bilateral shell-calcifications instead of a single central one;
or, like _Anodonta_, they may develop within the gill-plates of the
mother, though without presenting such a specialized larva as the
glochidium. An example of the former is seen in the development of the
European oyster, to the figure of which and its explanation the reader
is specially referred (fig. 23). An example of the latter is seen in a
common little fresh-water bivalve, the _Pisidium pusillum_, which has
been studied by Lankester. The gastrula is formed in this case by
invagination. The embryonic cells continue to divide, and form an oval
vesicle containing liquid (fig. 24); within this, at one pole, is seen
the mass of invaginated cells (fig. 25, hy). These invaginated cells
are the arch-enteron; they proliferate and give off branching cells,
which apply themselves (fig. 25, C) to the inner face of the vesicle,
thus forming the mesoblast. The outer single layer of cells which
constitutes the surface of the vesicle is the ectoderm or epiblast.
The little mass of hypoblast or enteric cell-mass now enlarges, but
remains connected with the cicatrix of the blastopore or orifice of
invagination by a stalk, the rectal peduncle. The enteron itself
becomes bilobed and is joined by a new invagination, that of the mouth
and stomodaeum. The mesoblast multiplies its cells, which become
partly muscular and partly skeleto-trophic. Centro-dorsally now
appears the embyronic shell-gland. The pharynx or stomodaeum is still
small, the foot not yet prominent. A later stage is seen in fig. 26,
where the pharynx is widely open and the foot prominent. No ciliated
velum or pre-oral (cephalic) lobe ever develops. The shell-gland
disappears, the mantle-skirt is raised as a ridge, the paired
shell-valves are secreted, the anus opens by a proctodaeal ingrowth
into the rectal peduncle, and the rudiments of the gills (br) and of
the renal organs (B) appear (fig. 26, lateral view), and thus the
chief organs and general form of the adult are acquired. Later changes
consist in the growth of the shell-valves over the whole area of the
mantle-flaps, and in the multiplication of the gill-filaments and
their consolidation to form gill-plates. It is important to note that
the gill-filaments are formed one by one _posteriorly_. The labial
tentacles are formed late. In the allied genus _Cyclas_, a byssus
gland is formed in the foot and subsequently disappears, but no such
gland occurs in _Pisidium_.
[Illustration: FIG. 26.--Diagram of Embryo of _Pisidium_. The unshaded
area gives the position of the shell-valve. (After Lankester.)
m, Mouth.
x, Anus.
f, Foot.
br, Branchial filaments.
mn, Margin of the mantle-skirt.
B, Organ of Bojanus.]
[Illustration: After Drew, in Lankester's _Treatise on Zoology_. (A. &
C. Black.)
FIG. 27.--Surface view of a forty-five hour embryo of _Yoldia
limatula_. a.c, Apical cilia. bl, Blastopore. x, Depression where the
cells that form the cerebral ganglia come to the surface.]
An extraordinary modification of the veliger occurs in the development
of _Nucula_ and _Yoldia_ and probably other members of the same
families. After the formation of the gastrula by epibole the larva
becomes enclosed by an ectodermic test covering the whole of the
original surface of the body, including the shell-gland, and leaving
only a small opening at the posterior end in which the stomodaeum and
proctodaeum are formed. In _Yoldia_ and _Nucula proxima_ the test
consists of five rows of flattened cells, the three median rows
bearing circlets of long cilia. At the anterior end of the test is the
apical plate from the centre of which projects a long flagellum as in
many other Lamellibranch larvae. In _Nucula delphinodonta_ the test is
uniformly covered with short cilia, and there is no flagellum. When
the larval development is completed the test is cast off, its cells
breaking apart and falling to pieces leaving the young animal with a
well-developed shell exposed and the internal organs in an advanced
state. The test is really a ciliated velum developed in the normal
position at the apical pole but reflected backwards in such a way as
to cover the original ectoderm except at the posterior end. In
_Yoldia_ and _Nucula proxima_ the ova are set free in the water and
the test-larvae are free-swimming, but in _Nucula delphinodonta_ the
female forms a thin-walled egg-case of mucus attached to the posterior
end of the shell and in communication with the pallial chamber; in
this case the eggs develop and the test-larva is enclosed. A similar
modification of the velum occurs in _Dentalium_ and in _Myzomenia_
among the Amphineura.