530 



COMPARATIVE ANATOMY 



CHAP. 



V 



trachese are found in a few Pseudoscorpionidce and a few Cypliophthahnidce 

 (Gibbocellum, Fig. 372, s,). They show much similarity with the tracheae 

 of the Scutigera (p. 479). True spiral threads are not found in the 

 second and third forms of tubular trachese. 



Book-leaf traehese (tracheal lungs, lung sacs, Figs. 373 



and 374). The stigma leads into 

 a sac filled with air, into which 

 there project from the anterior 

 wall numerous leaves arranged like 

 those of a book. They are, how- 

 ever, also attached by their side 

 edges to the lateral walls of the 

 sac, so that the latter may be com- 

 pared with a letter-case divided by 

 many partition walls into numerous 

 compartments ; the walls of the sac 

 are internally lined with a chitinous 

 cuticle, a continuation of the outer 

 chitinous integument of the body ; 

 this is also continued on to the 

 leaves, so that these consist of two 

 somewhatcloselycontiguouslamellse 

 connected by (muscular ?) trabeculae 

 or transverse supports. Between 

 the two lamellse of a leaf the blood 

 enters 'from the ccelome and the 

 respiratory process takes place 



Fig. 373. — Longitudinal section through a 

 book -leaf trachea of an Araneld, diagram- 

 matic, after MacLeod, v, Anterior ; h, pos- 

 terior ; ye, ventral side of the boolc -leaf trachea ; 

 d, dorsal side ; 6c, integument of the ventral 

 body wall of the abdomen ; st, stigmatic aper- 

 ture : Illy air- or tracheal cavity ; (r, the spaces 

 between the tracheal lamellffi ; p, transverse 

 supports between the trachese. 



If we imagine that i 



through the lamellffi. 



Tlie most plausible view of the mor- 

 phological signification of these lung sacs 

 seems still to be that they are modified 

 tracheal tufts. If we imagine that in a tracheal tuft which opens outwardly 

 by means of a short tracheal trunk the separate tubules standing close together, 

 mutually flatten each other out into hollow plates, and that these hollow plates 

 become aiTanged in a row, we have before us a so-called book-leaf trachea or tracheal 

 lung. The separate very narrow spaces lying between the leaves of the air sac would 

 thus correspond with the lumina of the flattened tracheae. 



Ribbon-like flattened traohefe are in fact to be found in the Arancidm. Compare 

 further the figures of the tracheal tufts of Scutigera, p. 479, which gi'eatly facilitate 

 a comprehension of the view here given of the rise of book-leaf trachefe. 



Another view as to the morphological significance of the book-leaf trachese of the 

 Arachnoidea has been put forward by those who hold that the Arachnoidea and 

 especially the Scorpimiidce are nearly related to the Xiphosura. According to this 

 view the leaves or partition walls which project into the lung sac answer to the 

 branchial leaves of the abdominal feet of Limuhis, which have sunk below the 

 body surface. The 4 pairs of book-leaf tracheae in the Scorpimi would thus represent 

 rudiments of the 4 pairs of abdominal feet, i.e. of their branchial appendages. In 

 comparison with the view first given, this view seems to us artificial and unsupported 

 by comparative anatomy and ontogeny. 



