THE PHYLOGENY OF THE TRACHEAE IN ARANE.E. 
551 
phosis of tlie saccules into the tubules in the case of the first 
of the two methods given above, we further imagined the 
ventral part of tlie ante-chamber to lengthen slightly, we 
should have almost exactly the condition found in the ante- 
rior pair of tracheae of Capon ia, which differs from this ideal 
case only in two minor points, viz. in the pi-esence of trans- 
verse or spiral thickenings in the tubules instead of small 
spines, and of a medial group of tubules at the base of the 
ante-chamber. In fact, as I have shown above, the anterior 
pair of tracheae of Capon ia may be taken to represent the 
most primitive form of metamorphosed lung-books known in 
Avhich the saccules still persist as tubules. 
Diagram of a tracliea of Harpactes Hoinl)ergi (ad. J*), seen 
ill section. Magn. H14. ahd. ir. Alidomiiial branch of the 
trachea. ceph. tr. Ceplialothoracic trunk. s})". Siiiracle. 
spt. Anastoinosing spines, tub. Tubules. 
The second pair of tracheae in Capouiai, being wholly 
homologous with lung-books, plainly belong to the same type 
as the tracheae of the Dysderidac and Oonopidae, and are 
merel}^ somewhat more complicated by the duplication of 
eaich of the cephailothoracic trunks and the elongation of the 
aibdominal branch of the hitter. The simplest form of this 
tyjie, such as that found in Harpactes (text-fig. 19) and in 
Calculus (in both of which the anastomosing spines of the 
trunks still form ai simple network and do not bear a spiral 
thread or inner perforated tube), niaiy be easily derived from 
the anterior tracheae of Caponia by merely exaggerating the 
tubular elongation of the ante-chamber, already commenced 
