342 ALFRED GIBBS BOURNE. 
The third nerve also gives off another branch before it 
arrives at the inner seta line, which runs backwards and joins 
the branch of the first nerve of the next following segment. 
The three nerves of the lst segment which really arise 
from the subcesophageal ganglion appear to come off from 
the commissure. 
From about Segment xx onwards the second and third 
nerves arise so close together as to be indistinguishable one 
from the other, but the microscope shows distinct bands of 
fibres, and they separate after penetrating the body-wall. 
The histology of the ventral cord is dealt with in figs. 47, 48. 
The cord is covered throughout by flattened ccelomic epi- 
thelium cells. Under these is a layer of connective tissue 
in which occur large oval nuclei. Then follows a very thick 
sheath of almost hyaline substance in which are embedded the 
muscle-fibres; this shrinks away from the outer connective- 
tissue sheath in preserved specimens, as shown in the draw- 
ings ; a few scattered histological elements occur in it. 
' The muscle is present in enormous quantity as compared 
with other worms. It consists of longitudinally running cells 
placed either singly or in bundles. At the origins of the nerves 
these muscle cells run outwards to form a sheath to the nerve 
(fig. 48). 
With regard to the nerve tissue proper, there is a single 
giant-fibre only ; in the inter-ganglionic regions, the two nerve- 
cords are quite distinct from one another and from the giant- 
fibre; the inner and dorsal portion alone of each cord is 
occupied by longitudinally running nerve-fibres arranged 
more or less definitely in bundles; in the ganglia the two 
cords and the giant-fibre all form part of the same mass, the 
nerve-fibres retain here a similar position while the ganglion 
cells lie on the outer and ventral sides. 
Both the sections figured run slightly obliquely across the 
cord, which accounts for the facts that in fig. 47 one cord is 
slightly larger than the other, and that in fig. 48 the origin of 
a nerve is seen on one side only. 
