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.
| |
|
Fig. 26.—Diagram of Embryo of
Pisidium. The unshaded area gives
the position of the shell-valve. | |
| m, | Mouth. |
| x, | Anus. |
| f, | Foot. |
| br, | Branchial filaments. |
| mn, | Margin of the mantle-skirt. |
| B, | Organ of Bojanus. |
|
| After Drew, in Lankester’s Treatise on Zoology. (A. & C. Black.) |
|
Fig. 27.—Surface view of a forty-five hour embryo of Yoldia limatula. |
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.
Classification of Lamellibranchia
The classification originally based on the structure of the gills by P. Pelseneer included five orders, viz.: the Protobranchia in which the gill-filaments are flattened and not reflected; the Filibranchia in which the filaments are long and reflected, with non-vascular junctions; the Pseudolamellibranchia in which the gill-lamellae are vertically folded, the inter-filamentar and interlamellar junctions being vascular or non-vascular; the Eulamellibranchia in which the inter-filamentar and interlamellar junctions are vascular; and lastly the Septibranchia in which the gills are reduced to a horizontal partition. The Pseudolamellibranchia included the oyster, scallop and their allies which formerly constituted the order Monomyaria, having only a single large adductor muscle or in addition a very small anterior adductor. The researches of W. G. Ridewood have shown that in gill-structure the Pectinacea agree with the Filibranchia and the Ostraeacea with the Eulamellibranchia, and accordingly the order Pseudolamellibranchia is now suppressed and its members divided between the two other orders mentioned. The four orders now retained exhibit successive stages in the modification of the ctenidia by reflection and concrescence of the filament, but other organs, such as the heart, adductors, renal organs, may not show corresponding stages. On the contrary considerable differences in these organs may occur within any single order. The Protobranchia, however, possess several primitive characters besides that of the branchiae. In them the foot has a flat ventral surface used for creeping, as in Gastropods, the byssus gland is but slightly developed, the pleural ganglia are distinct, there is a relic of the pharyngeal cavity, in some forms with a pair of glandular sacs, the gonads retain their primitive connexion with the renal cavities, and the otocysts are open.
Order I. Protobranchia
In addition to the characters given above, it may be noted that the mantle is provided with a hypobranchial gland on the outer side of each gill, the auricles are muscular, the kidneys are glandular through their whole length, the sexes are separate.
Fam. 1. Solenomyidae.—One row of branchial filaments is directed dorsally, the other ventrally; the mantle has a long postero-ventral suture and a single posterior aperture; the labial palps of each side are fused together; shell elongate; hinge without teeth; periostracum thick. Solenomya.
Fam. 2. Nuculidae.—Labial palps free, very broad, and provided with a posterior appendage; branchial filaments transverse; shell has an angular dorsal border; mantle open along its whole border. Nucula. Acila. Pronucula.
Fam. 3. Ledidae.—Like the Nuculidae, but mantle has two posterior sutures and two united siphons. Leda. Yoldia. Malletia.
