THE PHYLOHENY OP THE TEACHE.E IN AEANE.E. 557 
drites of tlie pulmonary and tracheal segments are doubtless 
quite short. If now the tracheal spiracle moved to the 
hinder end of the body and the entapophyses elongated 
correspondingly and became converted into a trachea we 
should get the type represented in text-fig. 21 (p. 554), which 
I consider to be the primitive type of all forms with meta- 
morphosed entapophyses. The anterior ends of the entapo- 
physes would still be near the iuterpulmonary fold, and the 
connecting muscular segment would still be quite short, as it 
always is in the spiders of group B. 
In Scytodes and Palpimanus the spiracle has moved 
to the hinder part of the body without any additional 
lengthening of the entapophysis. Hence in these two forms 
the segments of the longitudinal muscles belonging to the 
tracheal somite are greatly elongated, and iu this respect 
these forms (and allied genera) are apparently unique. 
Tlie Pholcidm, which Bertkau found to have no trachem 
at all, were perhaps derived from some form with a type of 
trachea similar to that of Pilistata, since according to 
Ijamy’s investigations a pair of entapophyses persists in some 
Pholcidse iu the same position iu which those of Pilistata 
are found (:02, pp. 191 and 192, figs. 32 and 33). The 
IMiolcidte, therefore, should perhaps belong, as regards the 
structure of their ninth somite (the tracheal segment iu 
other spiders), to the same group as the Pilistatidm, 
S i c a r i i d to, and P a 1 ]) i m a n i d m. 
In a previous paragraph two ])ossible solutions were 
suggested for the derivation of a tracheal system from lung- 
books, one of which appeared particularly applicable to the 
tracheal .system of the Hysderidm, etc. Now the second 
method suggested, which consists in the reduction of the 
respiratory functions of the lung-books by the abortion of 
the .saccules, appears to me to exactly meet the conditions 
found in the spiders with four simple tracheal trunks (or 
Avith two tracheal trunks and two entapophyses), iu Avhich 
the lung-books have numerous leaves, and obviously play the 
most important part iu the respiration. The size of the 
