152 On Vestigial Stigmata in the Arachnida. 



This comparison of Scorpio with Thely phonufi makes it also 

 evident that the tails in both tliese animals are not primitive 

 structm'cs. The tail is a later specialization, in Scorpio of 

 five segments, in Thelyphonus of three. This follows from 

 the fact that segments which in Scorpio form the tail, in 

 Thelyphonus are typical abdominal segments, unspecialized in 

 any way. 



Further, if it is possible to homologize the anal glands of 

 Thelyphonus, which open on each side of the soft circumanal 

 membrane and which are said to secrete formic acid, with the 

 poison-glands of Scorpio, and both these with the original 

 invaginations of the scar found on each side of the anal 

 papillffi in the Chernetidai, it follows that all these tail- 

 segments once possessed limbs with tracheal invaginations or 

 their homologues. We now know, indeed, from Thelyphonus 

 that the first tail-segment of Scorpio (the eighth) did actuall}- 

 at one time possess a pair of limbs [cf. figs. 2 and 3). 



As T have shown in my paper on the Galeodidaj (still ia 

 manuscript )y there is some evidence to justify us in believing 

 that the areas under discussion in Scorpio and Thelyphonus 

 represent limbs which have vanished. The primitive position 

 of the stigmatic aperture was probably just behind the coxa, 

 perhaps even on its posterior face. This is the position of the 

 thoracic stigmata of Galeodes with reference to the fourth pair 

 of limbs. If such a limb with a stigmatic aperture behind it 

 were to become rudimentary, it might either fold backwards 

 over the stigma, forming a kind of stigmatic operculum, or it 

 might simply flatten down, leaving the stigma free on the 

 sternal surface. Examples of both these processes may be 

 found in the Arachnida, not only within the same group, but 

 even in the same animal. For instance, in Thelyphonus the 

 functional stigmata open under opercula in the squeezed-up 

 anterior segments, whereas in the long segments their scars 

 are found along the posterior edges of the areas, which I take 

 to represent rudimentary limbs flattened down on the sternal 

 surface. In Scorpio the rudimentary limbs have been simply 

 flattened down, leaving the stigmata upon them. In the 

 Galeodidse in some genera the stigmata are under opercular 

 folds, in others the folds have flattened down, leaving the 

 stigmata exposed on the abdominal surface. 



In reference to the origin of these areas in the Arachnida 

 from rudimentary limbs which have disappeared by simply 

 becoming flattened down, it is interesting to note that the 

 stigmatic apertures in Scorpio very generally slope backwards. 

 In some the slope is very pronounced and is often parallel 

 with that of the pectines. It looks as if all these abdominal 

 limbs in Scorpio had once sloped backwards, as the posterior 

 functional limbs and the pectines still do. In some genera, 



