166 Macteay Menmoriat Vouume. 
9. A pair of external accessory longitudinal flexors, arising from the central 
muscle of the segment next but one in front and inserted with the preceding. 
10. A pair of odgue flexors, arising from the external flexors of the segment 
next in front. 
We thus arrive at the remarkable result that a typical segment in the abdomen 
of a macrurous crustacean has no fewer than ten pairs of muscles or muscular slips 
concerned in the simple movements of flexion and extension, of which alone it is 
capable. 
The following segments present variations from the typical arrangement just 
described :— 
Ist segment. 
1. The superficial extensor arises from the tergal region of the carapace. 
2. The deep extensor arises by five slips from the epimeral plate of the thorax. 
3. The superficial flexor arises from the sternal region of the thorax between 
the seventh and eighth segments. 
4. The place of the longitudinal flexors is taken by the thoracico-abdominal 
muscles which arise partly from the maxillary apodeme, partly from the 
epimeral plates. 
5. The oblique flexors and the accessory longitudinal flexors are absent. 
6. An additional external flexor is furnished by the outer limb of the enveloping 
muscle of the segment itself. 
7. The transverse muscle consists of three strands, one derived as usual from the 
central muscle, one from the outer limb of the thoracic enveloping muscle, 
and the third from slip No. 7 of the thoracico-abdominal muscles. 
2nd segment. 
1. The longitudinal flexor is constituted by the inner limb of the envelopmg 
muscle of the thorax. 
2. There are no accessory longitudinal flexors. 
3. The transverse muscle does not receive a slip from the enveloping muscle 
of the preceding segment. 
