178 MR. R. I. POCOCK ON THE * [June 17, 



true oral aperture, shows the inferior wall of the still wide, but 

 less strongly crescentic slit to be formed by a thickish transverse 

 horny plate which is described as the "persistent sternum of the 

 1st and 2nd segments" or the " supporting-rod of the labium." 

 Reference for comparison is given to a figure showing what 

 purports to be the same plate projecting forwards between the 

 coxfe when these segments are viewed from below, although the 

 figure showing the plate in transverse section represents it as 

 situated high above the lower surface of the coxae. This plate is 

 no doubt the thickened portion of the floor of the crescentic slit, 

 which is situated just in front of the true oral aperture ; but I 

 cannot establish any connection between it and the presternum 

 (sternum of the second postoral somite), or any part of the sternal 

 exoskeleton. 



Again, speaking of the "beak" of Arachnida, Bernard says {op. 

 cit. p. 391): — "The possession of this organ in such diverse 

 Arachnida as Galeodes, Chernes, and Thelyphonus, and the easy 

 deduction of the mouth-parts of Spiders, Scorpio and Phrynus, 

 from such an organ, renders it almost certain that a beak was 

 present in the original Arachnid." 



If the " beaks " of Galeodes, Ghernes, and Thelyj^homis were 

 similarly constructed organs, this argument would have weight ; 

 but, as a matter of fact, each of the orders represented by the 

 three Ai-achnids cited possesses a "beak" which is sui generis 

 and distinct from that of the other two, as well as from that of 

 all the other ordei's of the class \ 



Far more probable is it, in my opinion, that the " beaks" of 

 Thelyphonus, Galeodes, and Ghernes are deinvatives of mouth-parts 

 of a much simpler type, consisting primaiily of a camarostome or 

 prostomial labrum overhanging the oral aperture. So, too, from 

 this type can be deduced the very highly specialized "beak" of a 

 fourth kind which is met with in many Spiders, e. g., FiUstata, 

 Sicarius, &c. 



In fact, the types of mouth-parts characteristic of Scorpiones, 

 Thelyphonus, Phrynus, Galeodes, Pseudoscorpiones, Araneee, &c., 

 are all ti-aceable to one and the same simple plan of structure, 

 the modifications that are presented resulting from the formation, 

 one might almost say the necessity for the formation, of a suboral 

 trough to take up nutritive fluids. The one feature these mouth- 

 parts have in common is the labi'um or camarostome. In the 

 Scorpiones the suboral trough is formed by the sterno-coxal 

 (maxillary) processes of the third and fourth appendages (1st and 

 2nd walking-legs) (text-fig. 43, A, B, III, IV, p. 180). In Phrymis 

 it results from the basal vxnion beneath the mouth and the potential 

 approximation throughout their length of the preaxial surface of 

 the coxae of the appendages of the second pair (chelae or palpi). 



1 The diagrams representing transverse sections of the mouth-parts in Scorpio, 

 Obisiwn, and Galeodes, figured on pi. xxyii. figs. 9a-9c of Bernard's paper, 

 show very clearlj' the resemblances and differences and the true relations of the 

 organs. 



