nto two halves placed on
either side of the body. From each of these sub-oesophageal ganglia
numerous nerves arise. Passing from the middle line outwards they
are--(i.) the median pallial nerve to the middle of the dorsal mantle;
(ii.) numerous small nerves--the circum-oesophageal commissures--which
pass round the oesophagus to the chief arm-nerve or supra-oesophageal
ganglion; (iii.) the under arm-nerve to the lophophore and its
muscles; (iv.) the lateral pallial nerve to the sides of the dorsal
mantle. Laterally, the sub-oesophageal ganglia give off (v.) nerves to
the ventral mantle, and finally they supply (vi.) branches to the
various muscles. There is a special marginal nerve running round the
edge of the mantle, but the connexion of this with the rest of the
nervous system is not clear; probably it is merely another
concentration of the diffused sub-ectodermal nervous fibrils.

The above account applies more particularly to _Crania_, but in the
main it is applicable to the other Inarticulata which have been
investigated. In _Discinisca_ and _Lingula_, however, the
sub-oesophageal ganglion is not drawn out, but lies medianly; it gives
off two posteriorly directed nerves to the stalk, which in _Lingula_
unite and form a substantial nerve. Sense organs are unknown in the
adult. The larval forms are provided with eye-spots, but no very
specialized sense organs are found in the adult.

[Illustration: FIG. 28.--Diagram of nervous system of _Crania_; from
the dorsal side. The nerves running to the dorsal parts are white,
with black edges; those running to the ventral parts are solid black.
Magnified. (After Blochmann.)

1. Oesophagus.
2. Supra-oesophageal commisure.
3. Circum-oesophageal commisure.
4. Under arm-nerve.
5. Great arm-sinus.
6. Small arm-sinus.
7. Tentacle.
8. Lip of lophophore.
9. Infra-oesophageal commisure.
10. Chief arm-nerve.
11. Secondary arm-nerve.
12. Nerves to tentacles.
13. Sub-oesophageal ganglion.
14. Dorsal lateral nerve.
15. Sub-oesophageal portion of the secondary arm-nerve.
16. Median pallial nerve of dorsal lobe of mantle.
17. Anterior occlusor muscle.
18. Posterior occlusor muscle.
19. Obliquus superior muscle.
20. Levator brachii muscle.]

The _histology_ of Brachiopods presents some peculiar and many
primitive features. As a rule the