EESPIRATOEY ORGANS IN ARANEJl, 
85 
probably muscular tendons. They may have bad any other 
origin. Thus J. Wagmer (’94, p. 126), who has investigated 
the embryology of Acari and admits the branchial origin of 
hing-books, explains the cephalothoracic tracheae of Acari 
and Solpuga by deriving them from unicellular hypodermal 
glands, such as are found in water mites; while Borner 
(:02, pp. 455, 461, and 463) considers it very probable that 
the spiracle on the prosoma is that of the genital seg’ment 
displaced forwards. Further, Ray Laiikester’s suggestion 
that tracheae may have arisen by the tubefaction of meso- 
dermal strands may apply. 
The primitive nature of the lung-books in comparison with 
the trachem within the class Arachnida is in full agreement 
with the teachings of the comparative anatomy of other 
org’ans. Thus we 6nd only lung’-books in the highly seg- 
mented orders Scorpiones and Pedipalpi, tracheae in 
the orders with more concentrated bodies. Op i Hones and 
Acari, while in the Araneae the more primitive Tetra- 
pneumones have lung - books . only, the more highly 
specialised Dipneumones tracheae as well. The Solifugae, 
however, which are highly segmented, have tracheae only 
(see footnote on p. 84). 
Bernard’s theory that trachem have arisen from bristle-sacs 
of Chaetopod Annelids cannot be maintained for a moment as 
an explanation of the lung-books or trachem of Araneae in 
spite of the resemblance which the ectodermal tendons of the 
tracheal segment in my fig. 41 bears to the bristle-sac with 
its two oblique muscles figured by Bernard (’92, text-fig. 1, 
p. 512). Indeed, Bernard does not attempt to derive enta- 
pophyses from bristle-sacs, and the rudimentary spiracles 
(vestigial stigmata) which he claims to have found in Pseudo- 
scorpiones (’93a, p. 422, and ’93b, p. 26) and Pedipalpi 
(’94, p. 151) are always placed by him on the lateral side of 
the depressions caused by the dorso-ventral muscles of the 
abdomen. 
The homologies of the pulmonary segments in Arachnids. — 
On the accompanying page I have given a table representing 
