502 ZOOLOGY SECT. 



modification, by which their ends are converted into pincers or 

 clielce: 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 l \ 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 maxillipcd 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 antennse. The hindmost appendage 

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

 age : its protopodite (pr. 1, pr. %) is cut up into lobes comparable 

 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 (c-x), 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 exo- 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 

 fiagellum (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 antcnnule (1) has an axis of three podomeres (1-3) ending 

 in two many-jointed flagella (fi. 1. and ;'.'), which are sometimes 

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

 as we have seen, the exopodite springs from the second segment 

 of the axis, and the probabilities are that there is no exact corre- 

 spondence between the parts of the antennule and those of the 

 remaining appendages. 



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

 and are formed each of a small proximal and a large distal segment. 

 They are sometimes counted as appendages serially homologous 

 with the antennas, legs, &c. But, as we have seen in the case of 

 Apus, the appendages of Crustacea are always formed in regular 

 <)](!(. r from before backwards; the eye-stalks, on the other hand, 

 always appear later, both in individual development and in the 



