Interior views (generalized), examples of typical bivalve musculature in diverse dimyarian, heteromyarian, and monomyarian forms. (modified after Cox et al., 1969, Figs. 31-35). Pedal and pedalbyssal muscles in black, main pallial muscles shown as line, adductor muscle in stippled pattern. Where the adductor is shown as being divided into component muscles, the “quick” or striated muscle is unshaded, the “catch” or smooth muscle is stippled. Taxa and habitats as follows: Top row, relative size of catch and quick adductor muscle segments in A, epifaunal, pendent to closely attached, byssate
; B, epifaunal, free-living to weakly byssate Chlamys with swimming ability (note large “quick” muscle); C, cemented epifaunal
: D, slow burrowing, shallow infaunal
; and E, moderately rapid-burrowing, moderately deep infaunal
. Middle row, size and shape of foot, comparative position and size of pedal protractor and retractor muscles, and adductors, in diverse infaunal bivalves—I, slow-burrowing, moderately deep infaunal
: G, rapid-burrowing, moderately deep infaunal
; H, rapid-burrowing, shallow to moderately deep, mobile infaunal detritus feeder, Yolida; I, rapid-burrowing, deep infaunal
; J, strong but moderately slow-burrowing, shallow infaunal Neotrigonia, with most inflated shell of group. Bottom row, epifaunal to semi-infaunal, mostly sedentary suspension feeders—K, strongly byssate crevice and hard-surface dweller, moving shell to modify hard surface by grinding in some cases,
; L, semi-infaunal to (less commonly) epifaunal byssate
with moderately strong attachment; M, pendant, moderately to loosely attached, byssate
; N, free-living to weakly byssate Chlamys, with swimming capabilities; O, semi-infaunal, byssate
. Note relationship of muscle size, number and placement to habitat and behavior throughout. Key to letter symbols: apr, anterior pedal retractor; lg, ligament; LV, left valve; pbyr, posterior byssal retractor; pdm, dorsomedian pedal muscles; pe, pedal elevator; pl, pallial line; pp, pedal protractor; ppr, posterior pedal retractor; ps, pallial sinus; RV, right valve.
The periostracum, including that over the ligament, is secreted in
the groove between the outer and middle mantle layers, the outer
shell layer by the free edge of the outer mantle layer, and the
inner shell layer and ligament by the outer surface of the mantle
where it lies within the pallial line ( Fig.
The inner mantle layer bears small radial muscles which attach the
mantle to the shell—primarily in an arc around the mantle cavity
known as the pallial line but also within this line, with
clustering of small mantle suspender muscles commonly developed
dorsally ( Fig.
The position of siphons in bivalves is marked by a posterior
reentrant in the pallial line—the pallial sinus ( Fig.
Bivalvia have a complex system of muscles which operate the soft
parts, tie them together, and attach the animal to the protective
shell ( Fig.
The largest muscles are the adductors ( Fig.
In most primitive bivalves, and in virtually all modern infaunal
groups, the adductors are large, strongly attached, paired, and of
equal or near-equal size ( Dimyarian condition dimyarian ,
Isomyarian condition isomyarian condition ; Fig.
Semi-infaunal pinnaform and modioliform taxa and epifaunal
mytiliform bivalves commonly have the anterior adductor greatly
reduced relative to the posterior adductor ( Anisomyarian condition
anisomyarian condition ; Fig.
In bivalves that have taken on a cemented, byssate, or free-living
life habit which is basically recumbent on one or the other valve
(e.g., Ostreidae Ostreidae , Pectinidae Pectinidae ) the anterior
adductor muscle has been secondarily lost ( Fig.
Adductor muscles are divided into striated or “quick” muscle
bundles, and smooth or “catch” muscle bundles ( Fig.
Pedal and pedalbyssal muscles are usually concentrated dorsally
just inside dimyarian or anisomyarian adductors, under the umbonal
septum, and below the hinge line and dorsal margin ( Fig.
In addition, a large retractor may be located posteroventrally
adjacent to the posterior adductor muscle, as in Isognomon
3 , 4 ).
12, 31, 33, 34; Fig.
The size, strength, and direction of projection of the foot in
fossil bivalves can be indirectly interpreted from the size,
distribution, number, depth of impression, and buttressing of the
pedal protractor and retractor muscles ( Kauffman, 1969 ; Fig.
Reduction or loss of the foot is reflected by decrease in size and
number of pedal muscles and shell insertion areas ( Fig.
To a lesser extent, small visceral suspender muscles are inserted
dorsally on the inner shell surface ( Fig.
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