STUDIES ON THE ARACHNID ENTOSTERNITK. 255 



The one hypothesis of all others which, in my opinion, 

 has least in its favonr is that in the Solifugte we find a 

 primitive t^'pe of prosoma and a primitive type of entostei"- 

 nite clearly attesting the exoskeletal origin of this plate 

 in all orders of Arachnida ; and the conclnsions deduced 

 from these disputable premisses that the true entosternites 

 have been derived from chitinous integumental apodemes is 

 contradicted by their structure in the adult and their meso- 

 blastic origin in the embryo. 



Lastly, according to Bernard (3), the '^ diaphragm" of 

 Scorpio, "like that of Galeodes, is the homologue of the 

 great constriction between the sixth and seventh segments 

 forming the 'waist' of other Arachnids, , . . , a diaphragm or 

 waist being typical of Arachnids." It is not at all clear how 

 a partition like the Scorpion's diapliragm, composed of mus- 

 cular and connective tissue and without exoskeletal elements, 

 can be the homologue of on exoskeletal constriction. Ana- 

 logous structures, structures with the same physiological 

 significance, they no doubt are; but homologous, surely 

 not. 



According to Bernard's theory of waist formation, I pre- 

 sume the condition initiated in Tlielyphonus ;ind cul- 

 minating in the Spiders, a condition which results from the 

 constriction and reduction of the pregenital somite, preceded 

 the condition now met with in the Scorpions and Solifugse. 

 In that case the '' diaphragm " in these two orders must 

 represent the pregenital somite, insunk and overgrow^!, plus 

 the dorsal and lateral arthrodial membranes which connected 

 this somite with the prosoma in front and the mesosomabehind. 

 This double partition then became united into one, the dorsal 

 area of the pregenital somite disappeared, setting free the 

 aorta and the alimentary tract, which were previously confined 

 with the nerve-cord in a narrow canal, and enabling them to rise 

 and take up respectively their original positions in the dorsal 

 and central regions of the body-cavity, cleaving the partition 

 as they rose. Only by entertaining some such conception as 

 this is it possible to hold that the " waist " of the Spiders and 



