78 ARACHNIDA. 



opposition theory, concerning the origin of the lung-books, upon its absence. 

 Such a temporary retardation of one of the ontogenetic stages is here all the 

 less improbable, since we have seen that even the gills of Limulus appear some- 

 what sunk in the ventral body-wall. 



The most important evidence yielded by ontogeny in favour of a comparison 

 of the Arachnid lungs with the gills of the Xiphosura is their common origin in 

 connection with the abdominal limbs. Besides this, there is the striking agree- 

 ment in the structure of the adult organs which has been specially pointed out 

 by Ray Lankestee and MacLeod, and the canal-like communication of the 

 lung-sacs of the two sides which, to all appearance, finds its homologue in a 

 similar connection between the gill-cavities on the two sides in Limulus.* 



The Tracheae. If the pair of lungs in the Dipneumones arises 



from the second abdominal limbs, it must be assumed that the second 



pair found behind the first in the Tetrapneumones arises from the 



third pair of abdominal limbs. This pair, according to Morin's 



observations, disappears in the Dipneumones ■ but we may expect 



that further researches, made specially with this object in view 7 , may 



reveal that it gives origin to the two tracheal trunks w T hich are met 



with in most Araneids in addition to the lungs, t In a few Araneids 



(the genera Dysdera, Segestria, Argyroneta) the two stigmata of these 



tracheal trunks are found immediately behind the stigmata which 



lead to the lungs, and we can therefore hardly doubt that they are to 



be compared with the similarly-placed posterior stigmata of the 



Tetrapneumones. Where two stigmata (Salticus, Micryphantes), or, 



as is usually the case, a united pair of stigmata in the form of a 



transverse slit are found, immediately in front of the spinning 



mammillae, it might be assumed that the second pair of stigmata 



have been shifted back, just as the succeeding pair of limbs have 



been displaced backwards as the spinning mammillae to the posterior 



end of the body. 



Taking the above facts into consideration, we are inclined to trace back the 

 tracheae of the Araneae (and of the Arachnida generally ) to lungs, t We assume 

 that the air-chambers of the lungs lengthened out and extended far into the 



* [Simmons (App. to Lit. on Araneae, Xo. YIII.) finds that the lungs arise 

 as infoldings on the posterior surface of the second pair of abdominal appendages, 

 in the same way as the gills arise in Limulus, forming lung-leaves in the same 

 way as the gill-leaves are formed in the latter. The lung-leaves, therefore, arise 

 as an external structure on the posterior surface of the abdominal appendage ; 

 they sink in without any inversion or complication, in the way suggested by 

 Kingsley. The tracheae develop on the third abdominal appendage ; in their 

 earlier stages, these appendages have, on their posterior surface, a folding 

 similar to that on the second appendage. The author thus concludes that the 

 lung-book condition is primitive and the tracheae derived from it. 



See also Laltkie (App. to Lit. on Scorpiones, Xo. IV.). This author is also 

 in favour of MacLeod's view that the lung- books are to be derived from 

 appendages whose lateral margins have fused with the ventral body-wall. — Ed.] 



t [Pukcell (Xo. VII.) derives the inner tracheal trunk from an entapophysis, 

 the outer from the lung. — Ed.] 



