242 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 eosrmniitl 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. 
e 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 5a, 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 
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 
muscles are inserted in the base of this joint, which arise near the 
attachrnent 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. ‘he in- 
