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, (m, 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 
