h an object were required,
consequently none took rise from the lateral portions of the valves as
in _Lingula_; but in an extinct group, the _Trimerellidae_, which
seems to be somewhat intermediate in character between the Ecardines
and Testicardines, have been found certain scars, which appear to have
been produced by rudimentary lateral muscles, but it is doubtful
(considering the shells are furnished with teeth, though but rudely
developed) whether such muscles enabled the valves, as in _Lingula_,
to move forward and backward upon each other. _Crania_ in life opens
its valves by moving upon the straight hinge, without sliding the
valve.

The _nervous system_ of Brachiopods has, as a rule, maintained its
primitive connexion with the external epithelium. In a few places it
has sunk into the connective-tissue supporting layer beneath the
ectoderm, but the chief centres still remain in the ectoderm, and the
fibrils forming the nerves are for the most part at the base of the
ectodermal cells. Above the oesophagus is a thin commissure which
passes laterally into the chief arm-nerve. This latter includes in its
course numerous ganglion cells, and forms, according to F. Blochmann,
the immensely long drawn out supra-oesophageal ganglion. The chief
arm-nerve traverses the lophophore, being situated between the great
arm-sinus and the base of the lip (figs. 22 and 28); it gives off a
branch to each tentacle, and these all anastomose at the base of the
tentacles with the second nerve of the arm, the so-called secondary
arm-nerve. Like the chief arm-nerve, this strand runs through the
lophophore, parallel indeed with the former except near the middle
line, where it passes ventrally to the oesophagus. The lophophore is
supplied by yet a third nerve, the under arm-nerve, which is less
clearly defined than the others, and resembles a moderate aggregation
of the nerve fibrils, which seem everywhere to underlie the ectoderm,
and which in a few cases are gathered up into nerves. The under
arm-nerve, which lies between the small arm-sinus and the surface,
supplies nerves to the muscles of both arm-sinuses (figs. 22 and 28).
Medianly, it has its origin in the sub-oesophageal ganglion, which,
like the supra-oesophageal, is drawn out laterally, though not to the
same extent. In the middle line the sub-oespphageal nerve mass is
small; the ganglion is in fact drawn out i