364 



CRUSTACEA ENTOMOSTRACA. 



-en 



the trunk segments is sometimes traversed by longitudinal ob- 

 lique ridges, interpreted by Beecher as marking the apodemes 

 for muscular insertion. 



Beneath the head a large labrum (hy postoma] projects back 

 over the mouth, and a small lower lip is placed behind it. 



Appendages. A pair of long annulated antennae (Fig. 244), with 

 a large basal joint, are inserted on either side of the labrum. 



From the region of the mouth a series of appendages, set wide 

 apart, at the ends of broad sternites, extends to the end of the 

 pygidium. In the post-cephalic region they have, in Triar- 

 thrus, the following characters. They are biramous but peculiar 

 among crustacean limbs in being deeply cleft down to the 

 coxal process (Fig. 245). The endopodite is cylindrical in 



the anterior part of the trunk, 

 but its basal segments become 

 more and more lamellar as the 

 series is followed back. The exo- 

 podite consists of a long basal 

 segment and a multiarticulate ter- 

 minal portion, both beset with 

 an abundant fringe of long setae. 

 From the base of the limb what 

 appears to be a coxal endite ex- 

 tends inwards towards its fellow, 

 as in Apus and its allies ; but as, 

 apart from this, it is not clear 

 that there is any uncleft basal 

 portion of the limb (protopo- 

 dite), the homology of this element of the limb remains for the 

 present obscure. 



In the head Beecher concluded that there were four pairs of . 

 appendages in addition to the antennae, though, owing to the 

 difficulty in determining their points of insertion, the number 

 assigned to the head is in part a deduction from the number 

 of the segmental grooves on the dorsal surface of the glabella. 

 The two posterior head appendages appear to have had a 

 structure similar to that of the legs behind them, though with 

 a longer and denticulated gnathobase. The two appendages 

 in front of them had also, apparently, a large gnathobase, but 

 the conclusion that their appendages, like those of the segments 



FIG. 245. Dorsal view of the right 

 second and third thoracic legs of Triar- 

 thrus, as restored by Beecher. In 11. 

 the setae are omitted, en endopodite, 

 with segments 1-6 : e.i exopodite, with 

 segments 1-2. 7 the problematical 

 coxal process or gnathobase. 



