I [i]) 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.
This columella muscle is the same thing as the muscles adhering to the shell in Patella, and the posterior adductor of Lamellibranchs.
1, Posterior (pedal) adductor muscle of the shells.
1, A, B, mn), articulating at their bases with the head-capsule by sub-globular condyles, and provided with abductor and adductor muscles by means of which they can be separated or drawn together so as to bite solid food, or seize objects which have to be carried about.
F, foramen; d, deltidium; t, teeth; a, adductor impressions (= occlusors, Hancock); c, divaricator (=cardinal muscles, King, = muscles diducteurs principaux, Gratiolet); c', accessory divaricators (muscles diducteurs accessoires, Gratiolet); b, ventral adjustor (=ventral peduncular muscles, or muscles du pedoncule paire superieure, Gratiolet); b', peduncular muscle.
C, c', cardinal process; b', b', hinge-plate; s, dental sockets; 1, loop; q, crura; a, a', adductor impressions; c, accessory divaricator; b, peduncle muscles; ss, septum.
The bivalved carapace has a jointed rostrum, and covers only the front part of the body, to which it is only attached quite in front, the valve-like sides being under control of an adductor muscle.
M, End of adductor muscle.
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 FIG 22.
The former classification based on these differences in the adductor muscles is now abandoned, having proved to be an unnatural one.
In fact all Lamellibranchs begin with a condition in which there is only one adductor, and that not the posterior but the anterior.
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.
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.
- 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.
G, Anterior adductor muscle.
A,a, Anterior adductor muscle.
P.a, Posterior adductor muscle.
Anterior adductor muscle.
Posterior adductor muscle.
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.
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.
Ad, Adductor muscle.
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.
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 glochidium is formed by the precocious development of the anterior adductor and the retardation of all the other organs except the shell.
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.
Symmetrical, the anterior adductor small or absent.
The fresh-water Mussels, Anodonta cygnea, Unio pictorum, and Unio margaritiferus belong to the order Eulamellibranchia of Lamellibranch Molluscs, in which the anterior and posterior adductor muscles are equally developed.
The valves of the shell are closed by a single large adductor muscle, the anterior adductor being absent.
The organization of the oyster, as compared with that of a typical lamellibranch such as Anodon, is brought about by the reduction of the anterior part of the body accompanying the loss of the anterior adductor, and the enlargement of the posterior region.
The heart and pericardial chamber in the oyster lie along the anterior face of the adductor muscle, almost perpendicular to the direction of the gills, with which in Anodon they are parallel.
Modiolopsidae.-Extinct; Silurian to Cretaceous; adductor muscles sub-equal.
Pernidae.-Shell very inequilateral; ligament subdivided; mantle open throughout; anterior adductor absent.
Megalodontidae.-Shell 1a, tr, Upper and lower inequilateral, thick; posterior siphons adductor impression on a myo ms, Siphonal muscle of the phorous apophysis.
Mantle-flap. Megalodon; Devonian to Jur ma, Anterior adductor assic. Pachyrisma; Trias and muscle.
Adductor Jurassic. Durga; Jurassic.
Tridacnidae.-Mantle closed to a considerable extent; apertures distant from each other; no siphons; a single adductor; shell thick.
Cardiliidae.-Shell very high and short; dimyarian; posterior adductor impression on a prominent apophysis.