246 Description of a Species of Caligus. 



vides into two nearly equal portions, which diverge under the 

 large curved spine and pass to their attachment at the margin of 

 the shell. 



The base of the fourth pair of maxillipeds has a narrow pro- 

 longation, which affords attachment to two muscles ; one passes 

 posteriorly, and is attached near the articulation of the head and 

 thorax, (ra, fig. 7,) another extends outward in front, beneath the 

 extremity of the adjacent spine. Two other short muscles are 

 inserted at the base of the prolongation, and are also attached 

 near the spine ; one on the back, and the other below. The last 

 of the muscles moving this pair of legs, extends outward and is 

 attached to the epimeral articulation, (1, fig. 7.) 



The terminal claw is provided with flexor muscles of great 

 strength. A large conical muscle attached along the whole pos- 

 terior margin, is inserted in a bony tendon extending from the 

 inner portion of the base of the claw. Another large muscle 

 arises from the basal portion of the joint and is inserted into the 

 preceding muscle a short distance from its insertion. There is 

 the same arrangement in the female, (fig. 18 b.) A small extensor 

 is inserted in the outer part of the base of the claw and attached 

 to the outer posterior margin of the first joint. 



3. Anterior Thoracic Segment. — -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 

 digastric 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 in- 

 serted 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 articulation (n, fig. 7.) 

 This pair of legs, though thus provided with muscles of consider- 

 able strength, is seldom used by the animal in effecting its motions. 



We have already seen that the second pair of natatory legs 

 are well adapted to form powerful propelling organs; that the 



