258: MARINE INVERTEBRATES 
according to their external form. In Macrwra, the first subdivi- 
sion, belong the lobsters, crawfish, shrimps, prawns, and hermit- 
crabs, animals having a long and more or less cylindrical body, 
with the abdomen extended; in Brachyura, the second subdivision, 
are placed the crabs, animals having the thorax broad and flat, 
and the abdomen bent under. the thorax. The Decapoda have 
twenty segments, all of which, except the last one, have, at 
some period of life, a pair of appendages. The first two pairs of 
appendages, or, in the stalk-eyed forms, the first three pairs, are 
especially connected with the senses, and are often fringed 
with hairs, which are also considered to have a sense-function. 
The antennules, or first pair of appendages after the eye-stalks, 
are sometimes divided into two or three branches. At the base 
of the antennules are the ears. The antenne, or second pair of 
appendages, are undivided, but are larger than the first pair, and 
are often very long. At the base of the antenne are the renal 
glands. Both the antennules and the antenne are slender, elon- 
gated, movable, and full of joints. In some species they are greatly 
modified, as in Scyllarus, where they are developed into broad 
swimming-plates and, perhaps, as shovels for burrowing; in some 
amphipods they are used as swimming-organs. (Plate LX.) 
The next six pairs of appendages are grouped about the mouth. 
They are the mandibles, the mazxillw, and the mavillipeds. The 
mandibles are at the mouth-opening, and, being heavy and hard, 
are adapted to tearing and grinding; they have a jointed attach- 
ment, the palpus, whose office is to keep the mandibles clean. 
The two pairs of maxillw are delicate and leaf-like. The three 
pairs of maxillipeds grow gradually larger, the last pair being 
very prominent and extending over the other mouth-parts. Next 
come five pairs of walking-feet. One or more pairs of these feet 
have pincer-like ends, or claws. Some species have the claws im- 
mensely developed, as in lobsters. The claws are the chelw, and 
the feet which bear the chele are termed the chelipeds. The rest 
of the walking-feet have generally single, hook-like ends, but are 
variously modified in different species. The abdominal segments 
have six pairs of appendages, also variously modified. The last 
segment is without appendages, but often is extended into a tail, 
