314 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
NERVOUS SYSTEM. 
The nervous system consists of a ring of nerve fibres 
and ganglion cells, which encircles the pharynx (Pl. IIL., 
fig. 28, pp.n.r.) close to the mouth, and five radial trunks 
(Pl. III., fig. 28; Pl. IV., figs. 29 amd “aos eenrer 
traverse the entire length of the ambulacra, upon the 
inner surface of the test, and in close relation with the 
radial canals of the water-vascular system. From the 
peripharyngeal ring, in the radii, nerves are given off to 
the oral integument (PI. IT1., fig. 28, 2.h.) and the wall of 
the pharynx, in which they break up to form plexuses. 
In traversing the ambulacra the radial trunks give off, 
at intervals which correspond with the lateral branches of 
the water-vascular canals, nerves which pass through the 
ambulacral pores (Pl. IV., fig. 29, l.n.), and are distributed 
to the tube-feet and the periphery of the test. In both 
cases the fibres become continuous with those of the 
sub-epithelial nerve plexus. In a sagittal section of the 
distal end of a radial trunk (Pl. IV., fig. 30, 7.1.) this 
intercommunication of its fibres with those of the sub- 
epithelial plexus is specially obvious. In transverse 
section the peripharyngeal ring and radial trunks present 
the form of broad bands (figs. 26, 29, and 33). In ultimate 
structure they consist of ganglion cells, of which a well- 
marked layer lies upon the peripheral surface, 7.e., the 
surface nearest to the test, and nerve fibres, which run in 
a longitudinal direction, and amongst which a few ganglion 
cells are scattered. 
In addition to the peripharyngeal nerve-ring and radial 
trunks, there are five lamelle, consisting of ganglion cells 
and nerve fibres, which lie in close proximity to the nerve- 
ring at the points of origin of the radial trunks. From 
each lamella a pair of large nerves arise. These ascend 
