x PHYLUM ARTHROPODA 217 



form of broad flat plates : in the natural position of the 

 parts they lie one on each side of the telson, forming with it 

 a large five-lobed tail-fin : they are therefore conveniently 

 called uropods or tail-feet. The telson itself bears no 

 appendages. 



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 chefa, and hence 

 called chelipeds (Fig. 114, #). The three anterior segments 

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

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

 are distinguished as maxillipeds or foot-jaws (Fig. 115, 5-7). 



The structure of these appendages is best understood by 

 a consideration of the third viaxilliped (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 (^jf.), 

 and from the first springs a thin folded plate (ep.\ 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. 1, 

 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 



