104 ISOPODA. 



rally nearly equal in size,* movable, and with the pos- 

 terior lateral angles generally extending backwards over 

 the base of the legs. The large dorsal arc of each seg- 

 ment is accompanied at each side with a small but distinct 

 plate (epimeron of M. Milne Edwards), on the under- 

 side of which the corresponding leg is affixed. These 

 scale-like pieces, which are the first joints of the legs, 

 are often soldered to the dorsal arc, their limits being 

 indicated by impressed lines ; but in other cases, as in 

 Rocinela, -ZEga, &c., they are very distinct, forming 

 acute lateral points to the segments. The legs are, for 

 the most part, of small size, and more or less uniform 

 in shape : they are (generally) affixed to the lateral 

 margins of the underside of the body, the basal joint 

 being long and lying flat on the breast, its extremity 

 being brought into contact with the extremity of the 

 basal joint of the opposite leg, whilst the second and fol- 

 lowing joints of the leg are affixed at an angle and 

 directed outwards. This arrangement of the limbs is 

 especially marked in the anterior pairs, which thus put 

 on the appearance of outer mouth-organs. The legs 

 generally terminate in hook-shaped fingers, which, espe- 

 cially in the parasitical species, are of considerable size 

 and power. In the arrangement of these limbs there 

 may, however, notwithstanding their general conformity, 

 be observed somewhat of the division into series which 

 we have alluded to in our introduction to the Amphipoda 

 (vol. i. p. 7). Thus in Arcturus the first pair, the second, 

 third and fourth pairs, and the three posterior pairs, con- 

 stitute distinct series both in structure and function, 

 whilst in Serolis and Apseudes the first pair is as greatly 



* In Tanais, which in some respects might almost be regarded as the link 

 between the Isopoda and the Squillidse, the first segment of the body is of 

 large size, and is confluent with the head. 



