THE PHYLOGENY OP THE TRACHEA IN ARANE^. 523 
which the primitive lung-books have been retained, whereas 
they have been replaced by tracheae or lost in all other 
members of the group. It is not at all necessary to assume 
that the Hypochilidse once possessed tracheae, nor that the 
lung-books and the tracheae must necessarily be equally 
primitive organs. 
Lamy’s conception of a lung-book as developing from an 
ectodermal invagination with lamellate branches (p. 256) is 
incorrect, since, as I had already shown some years previously 
(’95), the two oldest saccules^ are formed as independent 
invaginations on the free posterior side of an embryonic 
abdominal appendage, quite outside of the basal pulmonary 
sac (vestibule) in the anterior wall of which the remaining 
saccules appear, — the two oldest saccules being only later on 
included within the pulmonary sac, when the sinking of the 
appendage takes place. Lamy puts this observation aside 
with the remark that I am the only observer who mentions it, 
but it is none the less a fact. 
I have already (:09) fully discussed the question of the 
primitiveness of the lung-books, and have shown on purely 
embryological grounds that the typical form of trachem found 
in most spiders must have been derived in part from lung- 
books and in part from ectodermal tendons (entapophyses of 
Eay Lankester, apodemes), the lateral pair of tracheal trunks 
being metamorphosed lung-books and the medial pair meta- 
morphosed entapophyses. 
Starting from this as a basis, the tetrapiieumonous group, 
Araneso theraphosse, appears the most primitive of living 
Araneas, a view which has, in fact, long been generally 
recognised on account of other primitive characters of the 
group, such as the presence of a free nervous ganglion 
behind the central nervous mass in the cephalo-thorax, the 
I have given the term “saccules” to the hollow air-containing 
leaves of a lung-book and “septa” to the j)artitions or lamellaj 
separating the cavities of adjacent air chambers (: 09). 
