1326 



CRUSTACEA. 



flexed by two short slender muscles on each side (R, R'), situated just 

 exterior to the process which forms the articulation of this segment 

 (figs, la and h, PL 93), and directed backward and outward. They 

 unite in a common short tendon. They act in depressing this seg- 

 ment, and assist in attaching its cup and anterior margin. This mar- 

 gin is provided with a narrow ridge, which is striated or wrinkled 

 transversely, like the cup, and is apparently intended to produce a 

 closer attachment of this margin. 



For the motions at the medial articulation of the cephalothorax 

 there are three pairs of muscles, situated in the anterior segment, two 

 attached near the median line, and one pair laterally. A pair of 

 short muscles (I, I, fig. h), run nearly parallel with the median line; 

 they produce the slight flexion admitted at this articulation. Another 

 pair of muscles, long and large (S), are situated on each side of the 

 preceding; they pass obliquely outward. In addition to aiding in 

 flexion, they produce a lateral sliding motion, often observed between 

 these segments. A third pair (K) also assist in flexion. The large 

 muscles (K') situated in the posterior segment, appear also to pertain 

 to this joint ; but we are not fully assured that this is really their 

 insertion. 



The extensor muscles of the posterior thoracic segment, and of the 

 abdomen, arise adjacent to the median line, near the centre of the ante- 

 rior thoracic segment. Three pairs of muscles are attached at this point. 

 The outer (L) pass obliquely outward, ancj. are inserted near the apex 

 of the posterior thoracic segment. The two pairs (M, N) appear to 

 continue through the thorax, to the last joint of the abdomen. Ano- 

 ther pair of muscles (0) commence in the thoracic joint, near the 

 median line ; they pass obliquely outward to a point in the first abdo- 

 minal segment, just below its centre, where they are inserted into the 

 teguments. Another pair of slender muscles (P) arise near the inser- 

 tion of the last, and pass to the following segment. 



The flexor muscles of these segments, situated along the venter, are 

 remarkable for having but two anterior attachments, although, count- 

 ing 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. If, the large muscle here continuing on, much dimi- 



