COMPAEATIVE MOEPHOLOGT OF THE GALEODID.E. 375 



Other Araclmids liave tubular tracheae, hut none closely resembling those of Galeodes. As a rule the 

 tubular trachete arise as tufts of exquisitely fine tubules from the swollen end of a simple tracheal 

 invagination. These tubules do not appear as a rule to anastomose one with another. This tuft- 

 arrangement is a second distinct specialization of the primitive tracheal invagination. 



A third equally explicable development of the primitive simple invagination is the so-called lung-book, 

 the origin of which was presumably as follows * : — As the alimentary diverticula became more and more 

 complex and distended, the inter-diverticular passages became closed. There was no mechanism which 

 we can see to keep tliem open. The blood, therefore, which originally flowed from the ventral sinus 

 up between the diverticula was compelled to flow out laterally, and so up between the skin and the 

 alimentary system ; in doing so, it would naturally run in the furrows which still marked off the 

 diverticula. This new direction of the blood-stream was not conducive to respiration, for instead of 

 flowing up the inter-diverticuLir passage alongside the tracheal (? branched) invaginations which 

 projected into this passage, it was now driven past the invagination at right angles to it. This defect 

 ■was rectified by the invagination developing flat leaf-like outgrowths arranged horizontally, between 

 ■which the blood, flowing out laterally, could pass. These lung-books are therefore secondary speciali- 

 zations of the more simple tracheie which once projected into the inter-diverticular spaces. In some 

 Spiders, the specialization has taken place only in one (viz., the anterior) pair, while the posterior pair 

 have persisted more nearly in their original form. The disappearance of the original inter-diverticular 

 space, except in the 2nd segment, and the development of a crowd of branching digesting tubules have, 

 in the Dipneumones, caused the simple tracheal tubes in the 3rd segment to develop as tufts of 

 tubules t- 



The diff"erent forms of tracheae have thus been closely dependent upon the variations in the alimentary 

 and circulatory systems. 



Where the alimentary diverticula simply closed up against one another, gradually shutting off the 

 channel up to the heart along the dorso-ventral muscles, the diverticula, however, still remaining fairly 

 distinct, the blood flowed from the ventral sinus outward laterally, and lung-books were developed, while 

 veno-pericardial muscles mark the remains of the old inter-diverticular passages, and are still so far 

 functional as to keep the passages open through which the blood flows out laterally through the lung- 

 books. 



When, again, the regular alimentary diverticula gave place to a confused mass of tubules without any 

 segmental arrangement, the original regular blood-stream was dispersed irregularly among the tubules, 

 and the tracheae grew out in all directions to follow it [Galeodes). 



In the Pseudoscorpions, on the other hand, we have quite a diff'erent arrangement ; on the squcezing-up 

 of the inter-diverticular blood-passages the respiratory tubules of the first pair grew forward in a dense 

 tuft into the cephalothorax, and thus met the ventral blood-stream flowing back through the body, while 

 those from the posterior pair grew backward and followed the stream (lo). 



I would thus deduce all the forms of trachea; from some simple form of chitinous invagination 

 occurring on each limb. It is obvious that the different existing forms cannot be deduced one from 

 another ; lung-books cannot develop tufts or the Galeodes system of lai-ge tubes, nor can either of these 

 produce lung-books. And yet there can be no doubt that the respiratory organs are all homologous 

 structures. The stigmata of Galeodes are obviously homologous with those of Thelyphonus and Scorpio, 

 inasmuch as their respective series overlap, and they occur in the same relation to limbs and in the 

 same relative positions on the limbs. We are compelled thus to assume some common structure from 



* On the supposed origin of these from invagiuated gill-plates see p. 398. 



t It is interesting to record th:it this method of esplaining the development of lung-books, arrived at by 

 -comparative morphology, has recently been confirmed by embryology. Jaworowski (35) has discovered that the 

 tracheal invaginations of Spiders first form branched tracheal tubes, and that the lung-books are a secondary 

 specialization. 



