5oS 
W. F. PURCELL. 
lung-books in FilisLata is not known, but judging from 
Lamy’s figure (:02, p. 172, fig. 11) they appear here, too, to 
be very large to compensate for the feeble development of 
the trachem. This relatively greater size of the anterior 
lung-books is exactly what 1 have explained should take 
place if the posterior lung-books became reduced to their 
ante-chambers only. 
If we imagined a tetrapneumonous spider with both 
pairs of lung-books connected by interpulmonary folds (the 
arachnomorphous spider Hypochilus appears to be such a 
form), and the entapophyses prominently developed in the 
second respiratory segment, as well as in the first, it would 
be perfectly simple to derive from it a form with ti-acheae 
exactly resembling those of Filistata. All that would be 
necessary would be that the saccules of the second pair of 
lung-books should disappear, leaving the two ante-chambers 
only; and that the two spiracles should come a little nearer 
together so as to form practically one opening with the 
intertracheal fold. It appears to me very probable that the 
tracheae of Filistata and of all other spiders (except the 
Dysderidm and tlieir allies) had this mode of origin, which 
is in entire agreement with the account given by Lamy of 
the structure of the tracheae in Filistata. The two short 
lateral tracheal sacs (p. 553, text-fig. 20, l.tr.) of this form 
are lined with spines and triangular in shape, exactly resem- 
bling a pulmonary sac deprived of its saccules. A study of 
the Hypochilidm would probably throw some further light 
upon this subject, since here the second pair of lung-books 
are placed, according to Simon’s figure ('Hist. Araign.,’ 
2nd ed., i, p. 201, fig. 145), far back, about midway between 
the anterior pair and the spinners, corresponding exactly in 
position to the tracheal system of the Filistatid le. 
I have made no attempt to explain the origin of those 
tracheal tubules, which cannot by any line of argument be 
derived from pulmonary saccules. The numerous tubules 
emitted from the large tracheal trunks in the Attidrn are a 
case in point, since these trunks, with the exception of their 
