EESPlKATOliY ORGANS IN AKANE^. 
77 
both the embryology and comparative anatomy, but 
to consider the medial trunks of the trachem as 
equivalent in their entirety to metamorphosed enta- 
pophy ses. 
It is, moreover, a common feature in the Arachnida for 
the ectodermal areas of attachment of various muscles to be 
invaginated into the body in the form of pouches or tubes for 
the purpose of serving as tendons, as, for instance, theentapo- 
physes (ec. t. 10 and 11, fig. 21) of the two following- abdo- 
minal segments already described. 
In order that an ectodermal tendon may become converted 
into a trachea it is only necessary that it should be hollow and 
sufficiently thin-walled, with free access of air to its interior, 
and that it should lie in blood or tissues requiring aeration. 
It is also evident that a tendinal trachea must have existed 
first as a simple entapophysis, since it could not possibly 
function as a trachea until after it had attained a tubular 
form. The entapophyses could not, therefore, have been 
originally produced for respiratory purposes. 
In the case of Araneae I have already sought to explain 
the elongated tracheal entapophyses by the great elongation 
of the ninth somite, and since the tubular entapophyses so 
produced are hollow and lie in the large ventral blood sinus 
(r. sin., figs. 41 and 43) we have here all the conditions 
necessary for their conversion into a trachea. For it is well 
known that the blood passes from this sinus to the lung-books 
and thence to the heart, and that the sinus, therefore, contains 
venous blood requiring aeration (Blanchard A9, ’50, Claparede 
’63, Schneider ’92, etc.). 
In the Tetrapneumouous spiders and in some Dysderidm 
(Segestria) we find the rudiments of the entapophyses of the 
ninth segment in the form of shallow depressions {ec. t. 9, 
text-fig. 2, p. o2) or pouch-like invaginations (ec. f. 9, fig. 32), 
already described on previous pages. These rudiments have 
no respiratory function, and if they were to approach near to 
the median line and be united at base by an iutertracheal in- 
folding we should obtain the conditions found in Filistata, 
