THE BRANCHIOPODA 39 



drawn in between them by the rhythmical movements of the 

 appendages in swimming. They are therefore distinguished as 

 gnathobases. The Branchiopoda are the only Crustacea in which 

 gnathobases are found on limbs far removed from the mouth. Of 

 the two exites, the distal, known as the flabellum (fl\ is a thin plate 

 with setose margins. It is moved by special muscles and probably 

 serves chiefly as a swimming-plate. The proximal exite, known as 

 the bract (In-}, is branchial in function, having a very thin cuticle, 

 without setae, and is not provided with muscles. The end of 

 the conn, external to the sixth endite, is produced into a rounded 

 process, the " sub-apical lobe." 



While most of the postmaxillary appendages of the Notostraca 

 have the general structure of that just described, differing only in 

 details of shape and proportion of the various parts, certain of them 

 are specially modified. In the first and second pairs, the conn is 

 divided, in the former into four, and in the latter into two segments. 

 The endites, with the exception of the gnathobase, become more 

 elongated in passing forwards along the series of limbs, and in the 

 first pair the second, third, fourth, and fifth endites, counting from 

 the base, are produced into filiform multiarticulate rami (Fig. 20, A). 

 The fifth, in some species, is nearly as long as the body of the 

 animal. The sixth endite, however, is much reduced and of peculiar 

 form in the limbs of the first pair. 



The trunk-limbs of the Conchostraca (Fig. 23, B) can without 

 difficulty be compared with those of Apus. The six endites are 

 distinct though reduced in size and less sharply marked off from 

 the unsegmented corm. The gnathobase is unprovided with 

 masticatory spines, but, like the following endites, is fringed with 

 setae. The fifth endite is produced in some cases into a long 

 tactile process (t). Only the sixth endite is marked off by a 

 distinct articulation. The flabellum is very large and the branchia 

 is reduced in size. 



In the Anostraca (Fig. 23, C, D) the structure of the parts is 

 rather different and their exact homologies are not quite clear. 

 The flattened unsegmented corm has its inner edge more or less 

 distinctly divided into six lobes. At the distal end, towards the 

 outer side, is a broad oval plate (fl) defined by a well-marked 

 articulation and fringed with setae. On the proximal side of this, 

 on the outer edge, is the branchia, characterised as such by its 

 thin cuticle and by the lack of setae. Still nearer the base the 

 outer margin of the corm is produced into a rounded plate (x), very 

 thin and not defined by an articulation. In Polyartemia and in 

 some species, at least, of Chirocephalus there are two such plates 

 (Fig. 23, D). It appears most probable that the six lobes of 

 the inner edge correspond to the six endites of the Notostracan 

 limb, in which case the distal setose plate (fl) will represent 



