THE EXTENSORS AND FLEXORS OF THE ABDOMEN. 99 
ride over the posterior edges of the branchiostegites. In 
the position of extension, the overlap of the terga is great, 
while that of the pleura of the middle somites is small. 
As the abdomen passes from extension to flexion, the 
overlap of the terga of course diminishes; but any de- 
crease of resistance to lateral strains which may thus 
arise, is compensated by the increasing overlap of the 
pleura, which reaches its maximum when the abdomen 
is completely flexed. 
It is obvious that longitudinal muscular fibres fixed 
into the exoskeleton, above the axes of the joints, must 
bring the centres of the terga of the somites closer 
together, when they contract; while muscular fibres 
attached below the axes of the joints must approximate 
the sterna. Hence, the former will give rise to extension, 
and the latter to flexion, of the abdomen as a whole. 
Now there are two pairs of very considerable muscles 
disposed in this manner. The dorsal pair, or the exten- 
sors of the abdomen (fig. 22, e.m), are attached in front 
to the side walls of the thorax, thence pass backwards 
into the abdomen, and divide into bundles, which are 
fixed to the inner surfaces of the terga of all the somites. 
The other pair, or the flexors of the abdomen (fm) consti- 
tute a very much larger mass of muscle, the fibres of 
which are curiously twisted, like the strands of a rope. 
The front end of this double rope is fixed to a series of 
processes of the exoskeleton of the thorax, called apode- 
mata, some of which roof over the sternal blood-sinuses 
H 2 
