ame authority, a pair of small muscles which have
their ends attached to the ventral valve, one on each side of the
median line, a little behind the united basis of the adductors, and
again to the extreme point of the cardinal process. Two pairs of
muscles, apparently connected with the peduncle and its limited
movements, have been minutely described by Hancock as having one of
their extremities attached to this organ. The _dorsal adjusters_ are
fixed to the ventral surface of the peduncle, and are again inserted
into the hinge-plate in the smaller valve. The _ventral adjusters_ are
considered to pass from the inner extremity of the peduncle, and to
become attached by one pair of their extremities to the ventral valve,
one on each side and a little behind the expanded base of the
divaricators. The function of these muscles, according to the same
authority, is not only that of erecting the shell; they serve also to
attach the peduncle to the shell, and thus effect the steadying of it
upon the peduncle. By alternate contracting they can cause a slight
rotation of the animal in its stalk.

[Illustration: FIG. 24.--_Magellania [Waldheimia] flavescens_. Diagram
showing the muscular system. (After Hancock.)

M, Ventral,
N, Dorsal valve,
l, Loop.
V, Mouth.
Z, Extremity of intestine,
c, Divaricators.
c', Accessory divaricators.
a, Adductor.
b, Ventral adjusters.
b', Peduncular muscles.
b'', Dorsal adjusters.
P, Peduncle.]

Such is the general arrangement of the shell muscles in the division
composing the articulated Brachiopoda, making allowance for certain
unimportant modifications observable in the animals composing the
different families and genera thereof. Owing to the strong and tight
interlocking of the valves by the means of curved teeth and sockets,
many species of Brachiopoda could open their valves but slightly. In
some species, such as _Thecidea_, the animal could raise its dorsal
valve at right angles to the plane of the ventral one (fig. 4).

[Illustration: FIGS. 25, 26. _Lingula anatina._

25, Interior of ventral valve.
26, Interior of dorsal valve.
g, Umbonal muscular impressions (open valves).
h, Central muscles (close valves).
i, Transmedial or sliding muscles.
b, Parietal band.
j, k, l, Lateral muscles (j, anteriors; k, middles; l, outsiders),
enabling the valve