I. CRUSTACEA: DECAPODA 377 



Order III. Decapoda. 



The Decapoda is the most important group of Crustacea, since it 

 contains the shrimps, lolisters, crayfisli, and crahs. It agrees with the 

 Schizopoda in having a cephalothorax composed of thirteen fused somites, 

 but differs in the structure and function of the thoracic extremities. Only 

 the last five pairs (whence the name Decapoda) are locomotor. These 

 lose the exopochte during development (Peneid;c excepted) and become 

 strong walking legs, terminated either with claws or pincers {chela). 

 Usually the first pair is distinguished from the others by its size and by 

 being chelate, and is a grasping organ. In the development of a chela 

 the penult joint sends out a strong process, the 'thumb,' which extends 

 as far as the last joint (the 'linger'), which closes against it. 



The mouth parts — a pair of mandibles, two pairs of maxillx, and three pairs 

 of maxillipeds (tig. 369) — lie in front of the first pair of legs. The maxiUipcds 

 (7, 6, 5) show a biramous condition, v.-hile the maxillse (4, 3] retain consideraljle 

 of the phyllopod character. In the mandibles (2) there is always a strong basal 

 joint, which serves as a jaw, while this may bear additional joints, the palpus. 

 Behind the mouth are a pair of scales, the paragnallis or mctastoma, which are 

 not appendages. The antenns are usually distinguished as antenns (second 

 pair) and anlenimlce (first pair, fig. 369). They have large basal portions, 

 which in the antennulas bear two many-jointed flagella, while the antennae 

 proper have but a single though usually much larger flagellum. On the basal 

 joint of the antennulaj is the otocyst (p. 362), while the green gland opens on 

 the basal joint of the antenna; (fig. 400, f;d). 



When the abdomen is not rudimentary (as in the crab.s) the appendages of 

 the si.xth abdominal segment together with the telson form a strong caudal fin 

 (fig. 400); the other appendages (fig. 369, /) are small, biramous organs to 

 which, in the female, the eggs are attached. In the female the first pair is 

 reduced, but in the male (except in Palinuridce) this pair is well developed, 

 curiously modified, and .serves as acopulatory (intromittent) organ. The sliape 

 of these appendages and the openings of the genital ducts — on the base of the 

 third walking foot of the female, the fifth in the male — serve at once to distin- 

 guish the sexes. Frequently also the males have the larger pincers. 



The thickness of the integument prevents diffuse respiration and 

 accounts for the numerous gills (fig. 398) which are attached to the bases 

 of the ma.xillipeds and walking feet or to the sides of the body near them. 

 (In the Thalassinidre the gills are on the abdominal appendages). These 

 gills are not visible externally, for the carapace extends^ do-\Mi on the 

 sides of the body as a fold (brandiiostcgite) over them, thus enclosing them 

 in a branchial chamber. A process of the second maxillrc — the scaphog- 

 nalhilc — plays in this branchial chamber and pumps the water over the 

 gills, the water flowing out near the mouth. All decapods can live some 

 time out of water; they retain some water in the gill chamber, which keeps 

 the gills in a moist condition. In some tropical oralis which live almost 

 exclusively on land there is a true aerial respiration, the lining of the 



