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.}| The mandibles are 
succeeded by two pairs of maxilla, 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., Pl. 12, fig. 17) represents the mandibular appen- 
dage of Spheroma as four-jointed, having mistaken a twist in the second 
joint for an articulation. 
+ The structure of the mandibles in Maia Squinado, and Nebalia, for ex- 
ample, affords strong presumption 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. 
