572 ME. H. M. BERXAED ON THE 



we shall see further, imply at least six pairs of inter-diverticular blood-passages through 

 which the blood once flowed upwards to the heart (PI. XXXIII. fig. 12). There is 

 evidence to show that in the primitive Arachnid a pair of respiratory invaginations 

 projected into each of these inter-diverticular spaces, aerating the blood as it passed. 

 The same argument would lead us to infer that there were at one time three pairs of 

 stigmata in the cephalothorax, as we have one aborted and two functional cardiac 

 chambers. The cephalothoracic stigmata persisting in the Galeodidse would be the first 

 of these pairs. 



A comparison o£ the stigmata of the Arachnida yields interesting results. 



In Thelyphonus, which has the longest heart with traces of ten cardiac chambers, nine being still 

 functional, there are no stigmata now in the cephalothorax, although, if we may judge from the heart 

 and the veno-pericardial muscles, there were certainly three pairs, as we infer that there were in Galeodes. 

 Along the abdomen we have two pairs of functional stigmata, and five pairs of stigmatic scars, reaching 

 back to the viiith segment. These scars occur in connection with faint vestiges of limbs closely resem- 

 bling the similar vestiges of limbs in Scorpio (15 and PI. XXVII. figs. 16, 17). Above the first four of 

 these scars (as above the functional stigmata) there is a blood-space held up by a veno-pericardial muscle- 

 strand. This arrangement implies that there was formerly an open passage up between the segmental 

 diverticula of the alimentary canal to the heart. The presence of scars on the viiith segment suggests 

 that there was at one time an extra [i. e., 11th) pairof ostia and veno-pericardial connections which have 

 now vanished. Even though the stigmatic scars no longer persisted, I should have felt justified in 

 concluding from these points in the internal anatomy of Thelyphonus that there had at one time been 

 stigmata on ten segments, from the 4th cephalothoracic to the viith abdominal (PI. XXXIII. fig. 12). 



While the limbs forming the genital opercula folded back on to the median line, those which followed 

 retained their lateral transverse positions ; we therefore have (in Thelyphonus) no approximation of the 

 stigmata such as we have in Galeodes {cf. PI. XXIX. figs. 11 and 13). 



In Scorpio there are seven pairs of ostia and seven pairs of veno-pericardial muscles, all, owing to the 

 great secondary compression of the cephalothorax, confined to the abdomen. These imply again that 

 there were once seven open inter-diverticular passages into each of which a tracheal invagination once 

 projected. Four pairs of invaginations only have been retained, viz., on the 3rd to the 5th segments. 

 The limbs to which the stigmata belonged, though not folding back into the median line like the genital 

 opercula, clearly sloped backward in various degrees (PI. XXIX. fig. 12). 



The Aranese oS'er, in some respects^ a curious parallel to Galeodes. There are never more than two 

 pairs of stigmata, viz., on the 2nd and 3rd abdominal segments. In some cases the posterior pair have 

 met in the middle line, which suggests that the limbs on which they occurred were also folded backward 

 near the middle. This is rendered stiU more probable from the fact that the spinning-mamillae, also 

 the remains of limbs, approximate in the middle line. 



In the foregoing cases we have in all traces of the remains of stigmata on 1 1 segments, 3 cephalo- 

 thoracic and 8 abdominal ; some still persist, some show merely as scars, others are only inferred 

 from the presence of cardiac chambers and veno-pericardial muscles. Taking these systems of organs, 

 respiratory and circulatory, together, with perhaps the series of dorso-ventral muscles which give rise 

 to the inter-diverticular passages, as, owing to their close physiological connection, we are justified in 

 <loing, we conclude that the primitive ancestral form of the Arachnida possessed 11 pairs of primitive 

 respiratory invaginations, each of which projected into an inter-diverticular blood-space. The blood, 

 in returning along the ventral floor of the body, flowed laterally and then up along the respiratory 

 invaginations through the inter-diverticular passage, entering the heart by the ostium situated above each 

 of these passages (PI. XXXIII. fig. 12). 



The various arrangements on which this scheme is founded may be best seen tabulated as follows : — 



