542 GEOFFREY SMITH. 
(cE) Muscular System. 
The dorsal muscles which lie above the pericardial space 
form four dorso-lateral, segmented, longitudinal bands running 
the entire length of the body. The oblique muscles are 
segmented bundles running obliquely downwards in each 
segment; they are very much larger in the abdominal 
segments than in the thoracic. Segmentally arranged ventral 
bands are also present, which again are larger in the abdominal 
than in the thoracic region. In order to see them and the 
nerve-cord in the abdomen, the oblique muscles have to be 
removed. 
(rF) Nervous System. 
We will deal first with the nerve-cord behind the sub- 
cesophageal ganglion. There are in the thoracic region 
eight distinct ganglionic thickenings, one for each free 
thoracic segment: similarly in the abdomen there are six 
ganglia (text-fig. 44). 
There are five bands of calcareous concretions situated 
between the first six thoracic ganglia (calc. text-fig. 44). 
This is the only place where lime is present in the whole of 
the body. 
Each thoracic ganglion gives off three chief pairs of nerves: 
an anterior thick pair which pass forwards to the appendages 
of the segment ; a more posterior slender pair which appear 
to supply the ventral muscles of each segment; and a still 
more posterior pair which innervate the oblique muscles. If 
we examine a single thoracic ganglion more carefully by 
means of sections, we may obtain a diagrammatic reconstruc- 
tion, such as is shown in text-fig. 45. The inter-ganglionic 
commissures (com.) are seen to fuse above and below the 
ganglionic area, but their fibres are continuous right through 
that area on the dorsal surface. The large nerves (N. 1) to 
the appendages send their fibres ventral to the commissural 
