168 Macieay Memoriat Vobume. 
the lateral region of the first abdominal segment of its own side. This latter move- 
ment may be assisted by the thoracic portions of the deep extensors, each of which, 
acting separately, would supplement the action of the rotator of the opposite side. 
2. Muscles of the cephalothorax. 
Owing to the concrescence of the cephalothoracie segments the muscles of this 
region are greatly reduced. They are, in fact, represented only by numerous more 
or less parallel fibres passing from the inner surface of the cervical groove to the 
inner surface of the anterior and dorsal regions of the epimeral plates (fig. 10), 
These fibres are described but not named by Huxley ; we propose to call them the 
tergo-epimeral muscles. Their action must be to effect a slight approximation of the 
carapace to the sterno-epimeral region of the thorax. 
Il. Tue Appenpicutar Musctss. 
The general arrangement of the intrinsic limb-muscles is well known, although 
often inadequately described. The typical arrangement is that each podomere or 
limb-segment is acted upon by two muscles situated in the preceding or next proximal 
podomere. The muscles are usually of unequal size, the larger being a flexor, the 
smaller an extensor. Both are bipinnate, their fibres arising from opposite sides of 
the segment in which they occur and passing obliquely into opposite sides of a strong 
chitinized or partially calcified tendon. The extensor tendon is inserted into the 
proximal edge of the podomere upon which it acts, on the extensor side, half-way 
between the two peg-and-socket joints. The flexor tendon is similarly inserted on 
the opposite or flexor side, so that a straight line joining the two insertions is at 
right angles to one joining the two articulations. This arrangement is shown in 
fig. 12, which represents the muscles of the fourth leg. 
The extrinsic limb-muscles, however, z.e., those which arise from the axial 
exoskeleton and move the appendage as a whole, vary considerably in the different 
kinds of limb. 
1. Extrinsic muscles of the legs (figs. 13-17). 
The two proximal podomeres of the leg—the coxo- and basipodite—are so short 
that the muscles acting on them may be considered as moving the limb as a whole. 
In accordance with the mode of articulation of these segments the flexor of the 
coxopodite acts as an adductor of the entire leg, its extensor as an abductor, the 
flexor of the basipodite as a levator, its extensor as a depressor. 
