x PHYLUM ARTHROPODA 219 



converted into pincers or chela ; the fourth segment (en. 4) 

 of the endopodite (sixth of the entire limb) is produced 

 distally so as to form a claw-like projection (en. 4'), against 

 which the terminal segment (en. 5) bites. The first leg is 

 much shorter than any of the others and its chela is of im- 

 mense size, and forms an important weapon of offence and 

 defence. The second maxilliped resembles the third, but is 

 considerably smaller ; the first (6) has its endopodite greatly 

 reduced, the two segments of its protopodite large and leaf- 

 like, and no gill is connected with the epipodite. 



The head bears a pair of mandibles and two pairs of 

 maxillae in relation with the mouth, and in front of that aper- 

 ture a pair of antennules and one of antennae. The hindmost 

 appendage of the head is the second maxilla (5) , a markedly 

 foliaceous appendage; its protopodite (pr. 1, pr. 2) is cut 

 up into lobes ; the exopodite (ex) is modified into a 

 boomerang-shaped plate, which, we shall see, is an impor- 

 tant accessory organ of respiration. The. first maxilla (4) is 

 a very small organ, having neither exo- or epipodite. The 

 mandible (j) is a large, strongly calcified body, toothed along 

 its inner edge, and bearing on its anterior border a little 

 three-jointed, feeler-like body, the palp. 



The antenna (2) is of great size, being nearly as long as 

 the whole body. It consists of an axis of five podomeres, 

 the fifth or last of which bears a long, flexible, many-jointed 

 structure, or flagellum (ft), while from the second segment 

 springs a scale-like body or squame (ex) . 



The antennule (1) has an axis of three podomeres (1-3), 

 ending in two many-jointed flagella (ft. 1 and 2). 



The eye-stalks, already noticed, arise just above the an- 

 tennules, and are formed each of a small proximal and a 

 large distal segment. They are sometimes counted as 

 appendages serially homologous with the antennae, legs, etc. 



