OF THE ACARlDiE TO THE ARACHNIDA. 289 



that in the adult forms, especially as this branching off of the 

 Acarida; from the Araneidas probably took place ages ago, when 

 the latter were themselves first specialized for their peculiar 

 manner of life. 



Hence we find that, as we go backwards from the specialized 

 Araneids with their book-leaf tracheae confined to the abdomen, 

 we come to the Acarids with purely tubular tracheae confined to 

 the thorax, according to my theory necessarily, because so little 

 of the abdomen is developed. 



The presence of these tubular cephalothoracic tracheae in the 

 Arachnida is not only no difiiculty, but is, on the contrary, 

 most natural if the Arachnida are classed with the other Tra- 

 cheata. The modern Hexapod has stigmata both on the thorax 

 and abdomen, while the primitive forms had stigmata on all the 

 thoracic segments, as is clearly seen in the embryos of many 

 forms. The typical Myriapod has stigmata on every leg-bearing 

 segment ; and Scolopendrella is said to have them on the head ; 

 and lastly Feripatus has them all over the body. The Mites, 

 then, in developing tracheae in the anterior part of the cephalo- 

 thorax have either retained the condition common to all primitive 

 Tracheata, or else have returned to it, owing to their failure to 

 develop a full number of abdominal segments. 



This simple explanation of the facts is in striking contrast to 

 the difficulties in the way of those who would deduce the Arach- 

 nidan tracheae from the embedded gills of the abdominal legs 

 of a Limulus-Yike ancestor. The only possible way to get over 

 the difficulty is to assume that tracheae have had four more or 

 less independent origins. In the Hexapoda they have had one 

 origin ; in the Arachnida two independent origins — the abdo- 

 minal book-leaf tracheae from embedded leg-gills, the thoracic 

 tubular tracheae in some other way again, while the abdominal 

 tubular tracheae are modified book-leaf tracheae. In one and 

 the same group the same structure, viz. the tubular tracheae, 

 have thus had two independent origins ! The more closely we 

 examine this subject, in fact, the more improbable does the gill- 

 origin of tracheae appear. We have, for example, in Ixodes ricinus 

 a pair of stigmata lying just behind the last pair of limbs. Until 

 more is known of the segmentation of Ixodes, it is not easy to 

 define the exact position of these stigmata ; but they belong either 

 to the last thoracic or to the first abdominal segment. However 

 that may be, it seems clear from their position that they are 

 closely related to the stigmata of the book-leaf tracheae lying 



