3C>4 



AKTIIROrODA 



abundant in the Silurian. The body (Ug. ,:;So) consists of Iiead and trunk, 

 the latter segmented. lu the ^"oung the segments are \-erv few, but increase 

 in number i_io-jo, according to the species) with age. The luiider seg- 

 ments frei|uenth' differ from the rest and form an abdomeli or pvgiJi/nii. 

 Dorsal!}' the animal is di\ided b\- U\'o groows into tliree lobes, marking 

 off in the head a i^Lil'flld anil two ,t,'<//ir.- in the trunk rliijiliis and two 

 plt:iii\c. Ow the head there are usuall)- a pair of compoimd eyes, ^^•lrich 

 in the voung were frei|uentl\' \eirtral. but are brought to the dorsal surface 

 with growth. Fur \-ears little was known of tlie ^■entral side, but latelv 

 specimens of Triarlr.nis hccki (lig. 3S1) from the I'lica slate ha\-e revealed 



\^M:,i?^)^'* 



v^ 



8"^^/ 



Fro. ^ 

 Fio. ,; 

 bears one 

 maxillx. 



Fig. 3S0. 



So. — Parailoxiih-s holuniiius i^froiu ZiUob. 

 Si. — Triarlhrus bctki. ventral surface, reslorei 



Fie.. ;Si, 



|..iir of antenna' anil four pairs of liirainous feel, the basal 

 Trunk n'uh birainous feel. 



(after Tieeeher). The head 



oints ser\\ni' as 



the appenikrges. On the liead are a pair of simple anlenn,T, and four 

 pairs of schizopodal feet, the bases of wlrich acled as jaws. It is a 

 question whether the first pair of ja.w feet ccu-respoiui to the second 

 antenna- or whetlrer these ]ra\e been lost in tire group. 'Fire trunk 

 segments bear biramous feet. In some respects the Irilobiles resemble 

 the Xijihosura (iiifni), but the possessioir of antenna' and biramous feet 

 place thenr among the Crustacea. Here their positioir is very imcertain. 

 We have little knowledge of but oire species, and this with its single 

 pair of antenn;e tlilTers from all recent Crustacea. 



