22 Mr. H. M. Bernard on 



Histological!} llie cndosternile proper of Scorpio sliows its 

 origin out of apodemes less plainly than does that of Gahodes. 

 This origin out of cuticular folds is, however, clear in cross 

 sections near their points of attachment to the body-wall 

 between the first and second legs ; in the other parts the 

 originally simple chitinous folds still visible in Galeodea 

 (fig. 2) have become changed, groups of cells having found 

 their way in between the chitinous layers *. Full of interest 

 is the marked difference between the cross sections of these 

 original and ancient apodemes forming the endosternite, which 

 have now become almost entirely detached from their parent 

 cuticle, and the recent secondary apodemes forming the coxal 

 endosclerites, which latter show their origin as cuticular 

 infoldings at a glance. These structures are well developed 

 in PJiryniis and Scorpio^ and they arise as an accompaniment 

 of the longitudinal compression of the thorax which charac- 

 terizes these two Arachnids. These apodemes remain con- 

 tinuously rigid with the posterior faces of the coxs. 



The endosternite of the ChernetidEe is now so specialized 

 that it will be difficult to ascertain its exact origin ; it appears 

 to be due to a fusion of segmental apodemes from between 

 the posterior thoracic segments, as it lies right back at the 

 posterior end of the thorax f. It may perhaps be homologous 

 with that of the Araneids. 



Enough has now been said to show conclusively that the 

 endosternites of the Arachnids are apodematous, and owe their 

 origin and varying forms to the different methods and degrees 

 of fusion and longitudinal compression of the cephalothoracic 

 segments. In Sco-pio this longitudinal compression of the 

 thorax is so pronounced that the endosternite has fused with 

 the diaphragm, and this has misled investigators into believing 

 that they belong to one another as parts of one and the same 

 structure. 



We may, then, safely conclude from this comparative survey 

 of the endosternites in Arachnids that Scoi-pio has not re- 

 tained the original condition of the Arachnidan thorax. But 

 this is after all in reality nothing more than what we can 

 learn from any comparative study of the Arachnids. We 

 have, for instance, every degree of fusion and longitudinal 

 compression of the first six ' segments, with Galeodcn at one 

 end retaining the largest number of segments unfusod, and 

 Scorpio at the other end with all the cephalothoracic segments 

 closely fused and compressed. 



* Cf. Lniikester (mitei), p. 18) mid Soliiniktwitscli (p. '20). 

 t* Cy. "Notes on the Cbometidio," Jomn. Liuu. Soc., Zoul. vol. xxiv. 

 p.;-J]0. 



