336 SCIENCE PROGRESS. 



behind the oenital aperture. This progressive disappear- 

 ance of posterior stigmata can still be followed in the 

 Galeodida;, where the whole aspect of the middle line of 

 the abdomen suggests the disappearance of a long series. 

 Three pairs are said to occur in the genus Datames.^ In 

 the genus Galeodes, the most posterior of these is degene- 

 rated to a single aperture, the tracheal tube to which 

 appears to be aborting and in some cases seems actually 

 closed ; while in the genus Rhax this last pair has finally 

 disappeared, leaving no trace either of aperture or tube. 

 The reason for this degeneration of the posterior stigmata 

 requires investigation. 



In the Book-scorpions there are only two pairs functional 

 with seven pairs of scars which I have claimed as the vesti- 

 ges of stigmata. In Thelyphonus, behind the two functional 

 stigmata, scars are found as far back as the eighth abdomi- 

 nal segment, these scars showing not only traces of the 

 former stigmatic apertures but also of the hard convex 

 scale-like plates to which the limbs associated with the 

 invaginations had been reduced. These plates are very 

 marked in Scorpio where they are still functional, and 

 serve no doubt to protect the specialised lung from ex- 

 ternal pressure. 



T/ie E^idoskeleton. — The remarkable endoskeleton which 

 is a characteristic of the Arachnida, and differs in each form, 

 has been the subject of a considerable amount of discussion. 

 The clue to its right understanding has again been supplied 

 by the Galeodidee. We gather from the condition of the 

 unfused abdominal segments in the Arachnida that originally, 

 i.e., in the ancestral form, all the harder rings of skin re- 

 presenting the segments were separated by very flexible 

 intersegmental membranes. In the abdominal regions 

 these are utilised in the formation of the collapsable vegeta- 

 tive sac. Between the sixth and seventh segments this 

 membrane has been drawn in to form the waist or diaphragm, 

 while in the region in front of the waist, on the harder 

 rings fusing together to form a rigid cephalothorax, these 



^ According to a figure by Putnam. 



