CALIGOIDEA. I33I 



(2.) Muscles moving the Natatory Appendages.— The two legs of each 

 pair of natatories have been described as simultaneous in their action, 

 which consists in their rotation with the included sternum, on their 

 anterior margin. 



The principal elevator of the first pair of natatories is a large digas- 

 tric muscle. This muscle occupies the space between the basal joint 

 of these legs and the preceding pair. It is composed of four muscles 

 which unite in a common tendon; this tendon passes under a curved 

 osseous process, by which it is confined in its place, and is then united 

 to another bundle of muscular fibres inserted in the lower surface of 

 the leg. The depression of these legs is produced by a long muscle, 

 which is inserted in the joint near its base ; it is directed forward and 

 outward, passing under the digastric muscle beyond the articulation 

 between the head and thorax, and is attached to the epimeral articu- 

 lation (n, fig. h). This pair of legs, though thus provided with 

 muscles of considerable strength, are seldom used by the animal in 

 effecting its motions. 



The second pair of natatory legs are especially adapted to form 

 powerful propelling organs ; the flabelliform arrangement of their pin- 

 nulae, the attachment of these pinnulae to two distinct articulated 

 branches, added to the flattened form of the joints, give the oars a 

 broad expanded surface for action on the water in swimming. They 

 are farther fitted for this object by the provision of a large number 

 of powerful 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. h) arise on 

 the median line— a third (q) at the median articulation of the cephalo- 

 thorax. Four powerful muscles are inserted in its posterior margin; 

 the three outer (u, t, s) pass backward, and are inserted in the poste- 

 rior 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 posterior margin of the leg. The cir- 

 cular 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 dissections. Two other short muscles, with con- 

 verging fibres (w, v), arise laterally from a broad base in the epimeral 



