ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 



611 



slits confluent along the middle line, and which give off a number of 

 ramifications ; the wall of each primary trunk is composed of an 

 external chitinogenous wall, and an internal chitinous layer, covered 

 by a large number of spines which unite with one another as in the 

 last tube of the lung. The author believes that the tracheae of 

 Argyroneta are nothing else than the last slit of the second lung 

 of Mygale, enormously developed, while the rest of the organ is 

 obliterated. 



A comparison between the lungs of Arachnids and the gills of 

 Limulus is then entered upon, in which especial attention is directed 

 to the work of Prof. Ray Lankester, which we have already noticed ; 



Fig. 110. 



Fig. 111. 



M. Macleod agrees in regarding the organs as homologues, but he 

 thinks that their relations may be explained more simply than they 



Fig. 112. 

 a b 



have been by the former. Let us 

 begin by supposing that there is 

 a considerable elongation of the 

 abdomen of the Limulus, without 

 any other change ; the result of 

 this modification would be that 

 the respiratory limbs would no 

 longer imbricate (Fig. 110). If, 

 now, the sternal region should en- 

 large and fuse for its whole length 



into the ventral, while the infero-superior axis were elongated, we 

 should arrive at the abdomen of a scorpion, where each lung was 

 replaced by a gill (Fig. 111). Each gill would be composed of a quad- 

 rangular plate fixed by its intimal edge A B, and its anterior edge B C 

 (Fig. 112a), and free on the other two ; this plate would serve for the 



