96 COMPARATIVE ANATOMY 



and the whole limb can be moved by the powerful muscles 

 which enter it and are attached to its walls (fig. 22, XVII). 

 Otherwise the axis is destitute of joints. The inner and outer 

 edges of the axis are produced into a number of lobes or 

 phyllites, of which six, borne on the inner edge, are called 

 endites, and three, borne on the outer edge, are called exites. 

 Of the six endites, that nearest the base of the limb stands 

 somewhat apart from the rest and is turned towards the middle 

 line. Its surface is covered with a number of stout setae, and 

 it serves, in conjunction with its fellow of the opposite side of 

 the body, to seize and conduct to the mouth any particles of 

 food with which it may come in contact. Examination of the 

 ventral surface shows that every one of the sixty-three pairs of 

 post-oral limbs of Apus is provided with a similar jaw-like pro- 

 cess or gnathobase, and the gnathobases of opposite sides 

 enclose a food-groove leading forward in the mid-ventral line 

 to the mouth. Though it has no definite joint, the gnathobase 

 is movable by means of special muscles inserted in the axis. 

 The four endites distal to the gnathobase are oval and leaf-like, 

 and are beset with setae arranged in a very characteristic 

 manner on the edges and sides, as shown in the figure. The 

 sixth or distal endite is a good deal larger and wider than the 

 others, and has a different arrangement of setae. It is attached 

 at the extremity of the axis, and extends both dorsally and 

 ventrally from its point of attachment. 



Of the three outgrowths from the outer edge of the axis, that 

 nearest the distal end is a somewhat wide expansion, fringed 

 with setae, but not provided with muscles. It is distinguished 

 as the sub-apical lobe. Of the two other exites the distal is 

 the largest of all the outgrowths of the axis. Its edge is beset 

 with stout spine-like setae, and it is furnished with three power- 

 ful muscular slips by means of which it can be moved to and 

 fro like a paddle. It is essentially a swimming plate, and is 

 known as the flabellum or fan. The proximal exite is a rather 

 thick oval outgrowth attached by a distinct pedicle to the axis. 

 It is not provided with any muscular slips, and is therefore 

 incapable of independent movement, but its internal spongy 

 tissue appears to contain numerous blood spaces. It is 

 probably respiratory in function, and o"n account of its passive 

 movements it is known as the bract (Lat. bractea, a weather- 

 cock). The postgenital limbs posterior to the iyth retain the 



