SCORPIONIDEA. 



801 



(iii) and (iv) have distinct gnathobases produced in towards 

 the mouth (Fig. 519) ; each walking leg has two terminal claws ; 

 between the bases of (v) and (vi) is a plate of varying out- 

 line, the sternum or metasternite, whose shape is of great 

 systematic importance. A segment belonging to the prosoma 

 exists in the young embryo (p. 775), but it disappears at an 

 early stage and leaves no appendages or indeed any other 

 evidence in the adult that it ever existed ; a similar segment 

 also disappears in Limulus (p. 789) ; we have followed Lankester 

 in omitting it from the enumeration of the adult segments. 



B. On the mesosoma : 



(vii) The genital operculum (Fig. 519), a foliaceous flap 

 with traces of a paired origin, which, when lifted up, reveals the 

 openings of the genital ducts, (viii) The pec tines, each with 

 a base bearing numerous processes like a comb ; the function 

 of these appendages is tactile. The appendages of segments 

 (ix), (x), (xi) and (xii) consist of a base bearing some 140 flat- 

 tened leaves which act as respiratory organs ; in the adult 

 they are contained in the pulmonary sacs, but in the embryo 

 they arise as external processes which later sink into a pit pre- 



e 9 



FIG. 520. Transverse section through the body of Euscorpim italicus in the region of the 

 endosternite and near the pectines (after Lankester). 1 Alimentary canal ; 2 caeca of gas- 

 tric gland (liver) ; 3 anterior aorta ; 4 endosternite ; 5 supraneural plate of endosternite ; 

 6 right ventral nerve cord ; 7 supraneural blood-vessel ; 8 chitinous tergum ; 9 chitinous 

 sternum ; 10 right coxal gland. The dotted areas represent sections of various muscles. 



pared just behind them ; had they remained outside the body 

 they would have looked exactly as the gills of Limulus look 

 (Fig. 512). 



-in 



3 F 



