78 
\V. V. PURCELL. 
etc. (text-fig. o, p. 72), and we have only to further imagine 
these entapophyses lengthened and to become thin-walled and 
provided internally with spines throughout in order to con- 
vert them into the tendinal tracheae of other Dipneumonous 
spiders. It is evident that the condition in the Tetrapueu- 
monous spiders, at any rate, is a primitive one, on account of 
the other primitive characters of this group, but the possibility 
of a reversion from an elongated tracheal tendon back to a 
very short one must be borne in mind, and may, perhaps, 
occur in some Dipneumonous spiders in which the ninth 
somite has secondarily become shortened again. I do not 
think that this has been the case in the Dysderidm, how- 
ever, on account of the primitive position of the tracheal 
spiracles (text-fig. 3, p. 69) and other primitive characters in 
this family, but in Argyroneta (text-fig. 4, p. 69) I believe 
there is evei*y probability that the spiracle was once more 
posterior and has subsequently shifted forward again to suit 
a newly acquired, aquatic habit. This would account for the 
fact that, although the actual tracheal entapophyses are 
extremely short, they are lined with the usual anastomosing 
spines and provided with a large medial outgrowth. This 
outgrowth may originally have been merely a medial pro- 
longation of the tracheal entapophysis beyond the ento- 
chondrite, and when the spiracle moved forwai-ds and the 
entapophysis shortened, its medial prolongation may have 
increased in inverse proportion, so as to maintain the effective- 
ness of the entire trachea as a respiratory organ. 
It would certainly appear that the tendinal trunks are more 
effective breathing organs than the lateral trunks are, pro- 
bably on account of the position of the former in the great 
ventral sinus of venous blood. For we frequently find the 
tendinal trunks v'ery strongly developed, and the lateral ones 
correspondingly reduced to a mere rudiment (Attidae) and 
sometimes apparently to vanish altogether (Argyroneta). 
The origin of the lateral tracheal trunks in Araneae. — I’he 
second question to be considered is whether the pair of lateral 
trachem of Dipneumonous spiders was derived from the second 
