STUDIES ON THE ARACHNID ENTOSTEHNITK. 245 



and by the fact that tlie soiuites in question have retained 

 their sternal elements, and are therefore, in that particular at 

 least, less modified than the somites in front, in which the 

 sterna have disappeared. If, then, Ave suppose that the 

 pharyngeal notch in the scorpions has been prolonged back- 

 wards almost as far as the position of the posterior transverse 

 bridge in Thel3'phonus, or as the corresponding thickened 

 ridge in the Aranea3 ; that the anterior three pairs of 

 apophyses have been suppressed upon the two cornua ; that the 

 lateral tendinous crests represent the similar crests in the 

 Araneas and Pedipalpi, those of Palamna3us in particular 

 recalling those of Tliely phonus, — it is evident that the ento- 

 sternites in the orders now under discussion are more alike in 

 reality than appears at first sight on the surface. Furthermore, 

 if we suppose that representatives of the muscles which radiate 

 from the margin of the posterior lobe of the entosternite in 

 Tliely phonus extended dorsally and laterally to meet 

 the walls and roof of the prosomatic space, leaving a channel 

 in the middle line for the transmission of the aorta and the 

 alimentary canal, it is possible to bring even the diaphragm 

 into harmony with parts already existing in the Thely- 

 phonus. In short, strip away the apophyses lettered Ity., 

 2tg., My., and sa.' in the figure ot" the entosternite of 

 Thelyphonus, remove the anterior bridge (a. h.), and fill up the 

 posterior foramen, and the liomology of the remainder of 

 the plate with the supra-neural portion of the entosternite in 

 the Scorpions becomes obvious. The annexation by the 

 entosternite of the tergo-sternal muscles of the genital somite 

 probably took place before the upgrowth of the posterior flap 

 shut off the cavity of the prosoina from that of the mesosoma ; 

 and this consideration points to the formation of the dia- 

 phragm after the suppression of the pregenital somite and 

 after the forward movement of the ventral area of the genital 

 somite, which brought its tergo-sternal muscle into contact 

 with the entosternite. 



The origin of the dorsal portion of the diaphragm in this 

 way from a pair of upgrowing muscular flaps embracing the 



