322 



COMPARATIVE ANATOMY 



CHAP. 



well-developed masticatory ridges. In contradistinction to the maxillipedes, the 5 

 posterior pairs of thoracic feet are described as ambulatory feet. The Decapoda, owe 

 to them their name. They are distinguished by the want of the exopodite, so 



that the appendage consisting of the 

 protopodite and endopodite is a simple 

 limb of 7 joiats. The exopodites, how- 

 ever, may be present in larval stages, 

 and in a few cases may be retained as 

 rudiments in the adult. The basal 

 joint of the ambulatory feet caiTy gills, 

 which project up into the respiratory 

 cavity. The Decapodan gills, however, 

 must be described in a special section. 

 The anterior ambulatory feet are often 

 chelate ; the first pair are generally very 

 powerful. In the cray-fish the 3 an- 

 terior pairs of ambulatory feet are 

 chelate ; the most anterior ambulatory 

 foot is provided with the well-known 

 large forceps. 



The Abdominal Feet (Pleopoda) 



(Fig. 225). 



The 6 anterior of the 7 ab- 

 dominal segments typically carry 

 limbs, while the last segment, 

 the telson, is always limbless. 



Arthrostraca. — In the Amphipoda, 

 setting aside for the present the aber- 

 rant group of the Caprellidce, we find 

 that the pleopoda are well developed 

 as typical biramose feet. The 3 an- 

 terior pairs of pleopoda are directed 

 forwards, and are strong swimming 

 feet with many -jointed exo- and endo- 



ex, exopodites ; 

 = podobranchiEe ; 

 branchiee. 



endopodites ; 

 arthrobranchiffi ; 



epipodites 

 pleuro- 



FiQ. 223.— Older larva of Oalllaxis in advanced 

 My sis stage. Gills and extremities of the cepbalo- 

 thoracic region (after Olaus). md, Mandible ; vixi, 

 anterior, mxj, posterior maxillae ; I-VIII, thoracic podites. The 3 smaller posterior pairs 

 feet, of ivhicli m/i = lst maxillipede, m/2 = 2d maxilli- of pleopoda (ff), also mostly biramose, 

 pede,m/3=3dmaxmipede,!,i-sr5, = ambulatoryfeet; „e directed backwards and pointed; 



they often serve for springing. The 



pleopoda are rarely lamellate. In the 



Caprellidce, where the abdomen is 



usually rudimentary, it carries at the most 3, at the least 2, much-degenerated limbs, 



which are better developed in the male (as copulatory feet) than they are in the 



female. 



Isopoda. — The pleopoda of the Isopoda are biramose limbs, whose endo- and 

 exo-podites are unjointed, and generally delicate - skinned, lamellae, which serve for 

 respiration. The last (6th) pair of pleopoda either forms together with the last 

 abdominal segment a rowing fin {F), as in the marine Isopoda, or is stylet-shaped, 

 as in fresh-water and land Isopoda. In the parasitic Bopyridce and Ci-yptoniscidce 

 the pleopoda are reduced or entirely wanting. In the land Oniscidm the outer lamellfe 



