Description of a Species of Caligus. 247 
flabelliform arrangement of their pinnule, the attachment of 
these pinnule to two distinct articulated branches, added to the 
flattened form of the joints, give the oars a broad expanded sur- 
face for action on the water in swimming. ‘They are farther 
fitted for this object by the provision of a large number of pow- 
erful muscles, which occupy nearly the whole of the thoracic 
segment. 
Inserted in the anterior part of these legs, there are three large 
muscles attached to the back shell, two of which (o, p, fig. 7) 
arise on the median line—a third (q) at the articulation between 
the head and thorax. Four powerful muscles are inserted in its 
posterior margin ; the three outer (u, t, s,) pass backward, and are 
inserted in the posterior and medial part of the segment above. 
The fourth (r) is attached to the back shell over the anterior part of 
the base of the leg, near the medial line of the body; it first 
passes inward and backward, then curves outward around the base 
of the muscle adjoining, (s,) and finally extends upward to the pos- 
terior margin of the leg. ‘The circular form of this muscle is so ~ 
very extraordinary, that we at first doubted its muscular nature. 
We have however assured ourselves of this fact by frequent dis- 
sections. 'T'wo other short muscles with converging fibres, (w, v,) 
arise laterally from a broad base in the epimeral articulation, and 
serve to retract the leg to the shell. These muscles probably co- 
operate with the posterior, in the depression of the leg. 
If these oar-like legs struck the water with the same broad ex- 
panded surface, in their backward motion, as in their forward pro- 
action, the animal would advance but slowly, if at all ; 
for the latter would be counteracted by the former. There is a 
provision against such a defect, in the muscles moving the several 
joints of these legs, by the action of which, the terminal portions 
receive a partial revolution, and cut the water, when drawn back- 
ward, by their thin anterior edge. Their special adaptation for this 
purpose is apparent, even in the pinnula terminating the leg, which 
instead of being ciliated on both edges, is furnished anteriorly 
with a thin membranous expansion. 
These legs appear to be the only organs for walking as well as 
ovale : 
The principal extensors of the third pair of legs, or the apron, 
are four in number ; two (y, z) arise on the back near the medial 
line, and pass laterally to the outer insertion of the apron. One 
