ISOPODA. 103 



loped, with the mandibles large and horny, the animals 

 being especially scavengers in their habits. The upper 

 lip is large and transverse, or somewhat triangular in 

 form ; the mandibles are generally subtriangular in form, 

 with a large prominent terminal dentated piece, below 

 which, in some groups, is a small secondary movable 

 denticulated appendage ; many of the species have this 

 organ furnished with a three-jointed* palpiform ap- 

 pendage, or rather (if we adopt the theory of these 

 mouth-organs being only modified limbs) the basal 

 horny part of the mandibles is composed of two or 

 three joints soldered together, whilst the terminal joints 

 are reproduced in the shape of a palpus, which is, how- 

 ever, obsolete in certain groups. -f- The mandibles are 

 succeeded by two pairs of maxillae, which are generally 

 of a delicate texture, the anterior pair composed of two 

 terminal subcylindrical curved lobes, with strong spines 

 at the tips, whilst the posterior pair consist of three deli- 

 cate membranous plates affixed to a basal stem. There 

 is, however, considerable difference in the structure of 

 these organs in the different groups. The pair of foot- 

 jaws closing the mouth beneath is generally of large size, 

 the basal portion often defended at each side by a large 

 triangular plate, whilst the terminal joints form a large 

 and broad palpus, shutting over the sides of the mouth, 

 which is, moreover, provided in its inner portion with a 

 bilobed lower lip of considerable size. 



The segments of the body, seven in number, are gene- 



* Savigny (Egypte Crust., PI. 12, tig. li) represents the mandibular appen- 

 dage of Sphteroma as four-jointed, having mistaken a twist in the second 

 joint for an articulation. 



t The structure of the mandibles in Maia Squinado, and Nebalia, for ex- 

 ample, aifords strong pi-esumption of the correctness of this view of the sub- 

 ject, the basal portion, although exarticulate, showing the three divisions of 

 which it is composed thoroughly soldered together. 



