243 Description of a Species of Caligus. 



are inserted near the apex of the thoracic joint. The two pairs 

 (M, N,) appear to continue through the thorax, to the last joint of 

 the abdomen. Another pair of muscles (O) commence in the 

 thoracic joint, near the median line ; they pass obliquely outward 

 to a point in the first abdominal segment, just below its centre, 

 where they are inserted into the teguments. Another pair of slen- 

 der muscles (P) arise near the insertion of the last, and pass to 

 the following segment. 



The flexor muscles of these segments, situated along the ven- 

 ter, are remarkable for having but two anterior attachments, al- 

 though, counting the several insertions in the posterior segments, 

 there appear to be six distinct muscles. Two broad muscles arise 

 on each side of the medial line opposite the prehensile legs. As 

 they pass between the sternums of the natatory legs, they divide 

 into three portions as represented in fig 5 a, PL I. The large 

 muscle here suddenly contracts in size, and afterwards continues 

 on, much diminished in volume ; exterior to this continuation two 

 muscles are attached, each by a tendon, to the diminishing portion 

 of the main muscle. Though apparently distinct, these three mus- 

 cles continue connected, and pass on beyond the sternum of the 

 second pair of natatories, where there is a second subdivison of the 

 muscle. We observe an oblique constriction of the whole, (fig. 

 5, b,) below which, the three muscles are continued of nearly 

 their former size, and a fourth is added, exterior to the three. 

 Thus divided, the muscle continues into the abdomen, where the 

 four parts are separately inserted: the exterior pair diverge and 

 are attached near the base of the abdomen ; the interior, are in- 

 serted below the centre of the abdomen, directly under the inser- 

 tions of the extensor muscles of the back ; the two remaining 

 pairs are continued into the terminal abdominal segment, the outer 

 passing beyond the centre of this joint. Another pair of small 

 muscles are inserted in the base of this joint, which arise near the 

 attachment of the interior pair of abdominal muscles. 



The other set of muscles, consisting of two pair, arise a short 

 distance below the sternum of the posterior natatory, exterior to 

 the muscles just described. One pair, the outer, is inserted in the 

 base of the thoracic joint, and the inner, laterally below the cen- 

 tre of the abdomen. 



The lateral motion of these joints is produced by the simultane- 

 ous action of the flexor and extensor of the same side. The in- 



