530 



COMPARATIVE ANATOMY 



CHAP. 



v-e 



tracheae are found in a few Pseudoscorpionidce and a few Cyphophthalmidce 

 (Gribbocellum, Fig. 372, s 2 ). 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 tracheae. 



Book-leaf tracheae (traeheal 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 

 somewhat closely contiguous lamellae 

 connected by (muscular ?) trabeculae 

 or transverse supports. Between 

 the two lamellae of a leaf the blood 

 enters from the ccelome and the 

 respiratory process takes place 

 through the lamellae. 



The most plausible view of tlie mor- 

 phological signification of these lung sacs 

 seems still to be that they are modified 



FIG. 373. Longitudinal section through a 

 book -leaf trachea of an Araneid, diagram- 

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

 terior ; ve, ventral side of the book-leaf trachea ; 

 d, dorsal side; fee, integument of the ventral 

 body wall of the abdomen ; st, stigmatic aper- 

 ture ; Ih, air- or traeheal cavity ; tr, the spaces 

 between the traeheal lamellae; p, transverse 

 supports between the tracheae. 



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

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

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

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

 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 tracheae are in fact to be found in the Araneidte. Compare 

 further the figures of the traeheal tufts of Scutigera, p. 479, which greatly facilitate 

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



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

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

 especially the Scorpionidce 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 Limulus, which have sunk below the 

 body surface. The 4 pairs of book-leaf tracheae in the Scorpion 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. 



