218 MANUAL OF ZOOLOGY SECT. 



The thoracic appendages are very different. The four 

 posterior segments bear long, slender jointed legs (8), upon 

 which the animal walks ; in front of these is a pair of very 

 large legs terminating in huge claws or chela, and hence 

 called chelipeds (Fig. 1 23, p) . The three anterior segments x 

 bear much smaller appendages, more or less leg-like in 

 form, but having their bases toothed to serve as jaws ; they 

 are distinguishable as maxillipeds or foot-jaws (Fig. 1 24, 6, 7) . 



The structure of these appendages is best understood by 

 a consideration of the third maxilliped (7). The main por- 

 tion of the limb is formed of seven podomeres arranged in 

 a single series, strongly calcified, and, with the exception of 

 the second and third, which are fused, movably articulated 

 with one another. The second podomere, counting from the 

 proximal end, bears a many-jointed, feeler-like organ (ex), 

 and from the first springs a thin folded plate (cp), having a 

 plume-like gill (g) attached to it. Obviously such an ap- 

 pendage is biramous, but with one of its branches greatly in 

 excess of the other ; the first two segments of the axis (pr. i, 

 pr. 2) form the protopodite, its remaining five segments 

 (en. 1-5) the endopodite, and the feeler, which is directed 

 outwards, or away from the median plane, the exopodite 

 (ex) . The folded plate (ep) is called the epipodite ; in the 

 natural position of the parts it is directed upwards, and lies 

 in the gill-cavity between the proper wall of the thorax and 

 the gill-cover. 



The five legs (8) differ from the third maxilliped in their 

 greater size, and in having no exopodite ; in the fifth or last 

 the epipodite also is absent. The first three of them have 

 undergone a curious modification, by which their ends are 



1 By most authors the maxillipedes are regarded as belonging to the 

 head, the number of pairs of thoracic appendages being considered as 

 five. AMERICAN EDITOR. 



