vn APPENDAGES 367 



lobes, while the exopodite is 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 an- 

 terior border a little three-jointed, feeler-like structure, 

 the palp, the two distal segments of which represent 

 the endopodite, its proximal segment, together with the 

 mandible proper, the protopodite. 



The antenna (2) is of great length, 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 feeler (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 feeler, the endopodite, and the squame the 

 exopodite. On the ventral side of the basal segment of 

 the protopodite is a conical elevation on which the duct 

 of the excretory organ opens (p. 375). 



The antennule (i) has an axis of three podomeres 

 ending in two many- jointed feelers (fl. i, fi. 2), which 

 are sometimes considered as corresponding to the endo- 

 podite and exopodite. But in all the other limbs, as we 

 have seen, the exopodite springs from the second seg- 

 ment of the axis, and the probabilities are that there 

 is no exact correspondence between the parts of the 

 antennule and those of the remaining appendages. 



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 

 and legs, but are more properly to be looked upon as 

 articulated processes of the prostomium. It is probable 



