xi PHYLUM ARTHROPODA 531 



(pr. 1, pr. 2) form the protopodite, its remaining five segments 

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

 wards, 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 (Fig. 446). 

 Its position is thus very similar to that of the flabellum of Apus, 

 while the gill attached to it is comparable to the bract. 



The five legs (S) differ from the third rnaxilliped 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 converted into pincers or 

 chelce : the fourth segment (en. 4) of the endopodite (sixth of the 

 entire limb) is produced distally so as to form a claw-like projec- 

 tion (en. 4 1 ), against which the terminal segment (en. 5) bites. The 

 first leg is much stouter than any of the others, and its chela is 

 of immense size and forms an important weapon of offence and 

 defence. The second maxilliped resembles the third, but is con- 

 siderably 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. 



As in Apus, the head bears a pair of mandibles and two pairs of 

 maxillae in relation with the mouth, and in front of that aperture 

 a pair of antennules and one of antennae. The hindmost appen- 

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

 appendage : its protopodite (pr. 1, pr. 2) is cut up into lobes com- 

 parable with the four proximal endites in the thoracic feet of 

 Apus : its endopodite (en) corresponds with the fifth endite, while 

 the sixth endite is represented by the exopodite (ex), modified into 

 a boomerang-shaped plate, which, as we shall see, is an important 

 accessory organ of respiration. The first maxilla (4) is a very small 

 organ, having neither exopodite nor epipodite. The mandible (3) 

 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 two distal segments (en. 1, en. 2) of which 

 represent the endopodite, its proximal segment (pr. 2) together 

 with the mandible proper (pr. 1), the protopodite. 



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 

 (fi), while from the second segment springs a scale-like body or 

 squame (ex). It is fairly obvious that the two proximal segments 

 represent the protopodite, the remaining three, with the flagellum, 

 the endopodite, and the squame the exopodite. 



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

 in two many-jointed flagella (fi 1, and 2), which are sometimes 

 considered as endopodite and exopodite. But in all the other limbs, 



M M 2 



