s to move forward and backward on each other.

(After King.)]

In the Ecardines, of which _Lingula_ and _Discina_ may be quoted as
examples, the myology is much more complicated. Of the shell or
valvular muscles W. King makes out five pairs and an odd one, and
individualizes their respective functions as follows:--Three pairs are
_lateral_, having their members limited to the sides of the shell; one
pair are _transmedians_, each member passing across the middle of the
reverse side of the shell, while the odd muscle occupies the umbonal
cavity. The _central_ and _umbonal_ muscles effect the direct opening
and closing of the shell, the _laterals_ enable the valves to move
forward and backward on each other, and the _transmedians_ allow the
similar extremities (the rostral) of the valves to turn from each
other to the right or the left on an axis subcentrically situated,
that is, the medio-transverse region of the dorsal valve. It was long
a matter in discussion whether the animal could displace its valves
sideways when about to open its shell, but this has been actually
observed by Professors K. Semper and E.S. Morse, who saw the animal
perform the operation. They mention that it is never done suddenly or
by jerks, as the valves are at first always pushed to one side several
times and back again on each other, at the same time opening gradually
in the transverse direction till they rest opposite to one another and
widely apart. Those who have not seen the animal in life, or who did
not believe in the possibility of the valves crossing each other with
a slight obliquity, would not consent to appropriating any of its
muscles to that purpose, and consequently attributed to all the
lateral muscles the simple function of keeping the valves in an
opposite position, or holding them adjusted. We have not only the
observations of Semper and Morse, but the anatomical investigations of
King, to confirm the sliding action or lateral divarication of the
valves of _Lingula_.

[Illustration: FIG. 27.--_Lingula anatina_.

Diagram showing the muscular system. (After Hancock.) The letters
indicate the muscles as in figs. 25 and 26.

A, Dorsal,
B, Ventral valve.
p, Peduncle.
e, Heart.
a, Alimentary tube.
z, Anal aperture.]

In the Testicardines, where no such sliding action of the valves was
necessary or possible, no muscles for suc