EESriRATORY OliGAXS IX ARAXEAi]. 
91 
sunken - in lung-books, or that the spinner - segments 
ever possessed such organs in any ancestral form of 
this o r d e r. 
When abdominal segments bearing spiracles in other 
Tracheate orders (Solitugae, Pseudoscorpiones, Opi- 
Hones, and Acari) are homologised with those bearing 
spinners in Araneae, as is done by Bbrner (;02, p. 457), the 
difficulty of deriving spinners from lung-books should be 
taken into account. For if the Lipoctena represent a 
natural group and the tracheae leading from these spiracles 
are derived from lung-books, as is often assumed to be the 
case, it follows that the spinners in Araneae must also have 
been derived from lung-books. But if we cannot admit the 
latter derivation, then either some or all of these tracheae are 
not homologous with lung-books (i . e . they are new forma- 
tions), or else the segments bearing them are homologous 
with the pulmonary segments in Araneae (and not with 
those bearing spinners), or, finally, some or all of these 
oi’ders maiy have originated independently of the Fed i palpi 
and Araneae from branchiate ancestors (whether in connec- 
tion with the Scorpiones or not is another question). 
In the fSolifugm two (or at least one) of the three 
tracheate segments of the abdomen must be homologous with 
segments bearing spinners in Araneae, and a knowledge of 
the development of the tracheae would be necessary before 
one could determine the relaitionships of this order. 
(4) Li m ulus. — According to Kingsley (’85) the genital 
segment in the American species of Li m ulus is the seventh 
post-onil segment, but Kishinouye (’91) has since discovered 
an additional somite between the last thoracic segment and 
the genital segment in the Japanese specie.s, thus making the 
latter segment the eighth post-oral one. It is possible that 
this aidditional seventh somite was overlooked in tlie American 
Limulus, just as it has fre(^uently been overlooked in the 
spiders and scorpions, for its presence would bring the 
segmentation of the abdomen of Limulus into line with 
that of spiders and scorpions. 
