Descriptio?i of a Species of Caligus. 247 



flabelliform arrangement of their pinnulas, the attachment of 

 these pinnulag 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. Two 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- 

 pelling 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 

 swimming. 



The principal extensors of the third pair of legs, or the ap?^on, 

 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 



