THE PHYLOCxENY OP THE TEACHER IN AEANE^. 559 
lateral basal lobes, are metamorphosed entapophyses. These 
tubules may have originated simply as outgrowths of the 
trunks, and would then, of course, be of ectodermal origiia. 
Ray Lankester is of opinion (:04, p. 223) that the tracheal 
tubules in Arachnida (and in all other Tracheata) have 
developed by adaptation of the vasifactive tissue of the 
blood-vessels,” which have come to open in the case of the 
Araclinids into the lung-chambers (and elsewhere). Instances 
of mesodermal tubes attaching themselves to, and opening 
into ectodermal invaginations are, of course, well known, e. g. 
the genital ducts. No actual embryological observations, 
however, exist, so far as I am aware, regarding the develop- 
ment of the fine tracheal tubules in Arachnida. In Attus 
floricola no trace of these tubules was found up to the stage 
formed at the second moult, and I had no later stages at my 
disposal.^ 
Summary. — The theoretical suggestions in the preceding 
paragraphs may be summed up as follows : 
In the first place I suppose the saccules of the second pair 
of lung-books to have been converted into tracheal tubules in 
the common ancestor of the Dysderidse, Oonopidse, and 
Caponiidae. The resultant trachem then increased in size, 
and, as the number of the leavms of the anterior lung- 
books decreased in inverse ratio, the former became the 
principal organs of respiration. The second pair of spiracles 
retained their position, or may even have moved slightly for- 
wards, and the conversion of the entapophyses into tracheae 
could not take place here, and would, moreover, be quite 
unnecessary. In the Caponiidae the anterior pair of lung- 
books were converted into tracheae in ai similar manner, but 
at a. later period, and independently of the conversion of the 
posterior pair; but as the latter already provided almost the 
' A paper by R. Janeck entitled “ Entwickluiig der Blattertraclieen 
and der Traclieen hei den Splnnen” lias recently a 2 ipeared ("Jena 
Zeitsclir. Naturw..’ xliv, Hft. 2 — 1, 1909), hut I have not hitherto had 
access to this iiublication. 
