258 MARINE INVERTEBRATES 



according to their external form. In Macrura, 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 antennce, or second pair of 

 appendages, are undivided, but are larger than the first pair, and 

 are often very long. At the base of the antennae are the renal 

 glands. Both the antennules and the antennae 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 maxillce, and the maxillipeds. The 

 mandibles are at the mouth-opening, and, being heavy and hare" 

 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 maxilla3 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 chelce, and 

 the feet which bear the chelffi 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, 



