Description of a Species of Caligus. 245 



close the esophagus, by means of a tendon inserted in the lower 

 extremity of these folds. 



The basal joint of the first pair of maxillipeds has but little 

 motion. There are two short muscles, elevating or depressing 

 the extremities of this joint, which we may consider a flexor and 

 an extensor. The flexor, which is inserted near the interior ex- 

 tremity, is directed backward and a little outward to its attach- 

 ment to the lower shell, exterior to the base of the following pair 

 of feet. The extensor is inserted at the posterior margin of the 

 joint, and extends obliquely inward, approaching the attachment 

 of the flexor. In the female these muscles have nearly the same 

 position as in the male (fig. 18 a;) the flexor is inserted near the 

 spine on this joint. The united action of these muscles draws the 

 anterior margin of this joint from the shell. To oppose this mo- 

 tion there is a large muscle inserted near this margin and extend- 

 ing one side below the eyes, (e, fig. 7,) where it is. attached to the 

 back shell. 



The extensor of the second joint of this pair of legs is a long 

 broad muscle attached to the shell above the large curved spine, 

 (f, fig. 7.) There is a small flexor of this joint, attached to the 

 posterior apex of the basal joint. 



The rudimentary feet, or second pair, are provided with but 

 few small muscles, requiring no remarks. 



The third pair are remarkable for having as various motions as 

 could be afforded by a ball and socket joint. This arises from 

 their insertion on a fleshy prominence, composed probably of the 

 rudiments of the small basal joints in the corresponding organs of 

 the higher Crustacea. To produce these various motions, each 

 leg is provided with five muscles radiating from the base, some of 

 which are of very peculiar form. Four of these muscles are in- 

 serted into the base of the first joint and one along its posterior 

 margin. The latter appears to be attached to the back near the 

 median line, a short distance behind the eyes, (g, fig. 7.) Of the 

 remaining muscles, two pass forward and outward, (h, i fig. I 

 and fig. 7,) one directly outward, and the fourth, (k, fig. 7,) back- 

 ward and outward. The most anterior (h) is a slender muscle, 

 attached just exterior to the base of the first pair of maxillipeds. 

 The second (i, fig. 1 and fig. 7) is composed of two parts in- 

 serted into the same tendon. These parts continue together 

 through half their length, then separate, and soon after each di- 



