RESPIliATORY ORGANS IN ARANE^, 
25 
oldest saccule is the fii'st to grow into the interior, while the 
others follow in turn in the order in which they were formed. 
Simmons (^94) is, so far as I am aware, the only author who 
has described and figured the earlier stages of the formation 
of the saccules in spiders. He gives two figures of sagittal 
sections from embryos of the same age, one (fig. 6) showing- 
five and the other (fig. 5) two pulmonary furrows. The 
position of these two furrows in the latter figure shows that 
they are not the two oldest, the others having apparently 
been missed by the section, which is probably of about the 
same stage as his fig. 6. Simmons’ account is as follows: 
The outer wall of the pocket “has its ectoderm thrown into 
folds,” the nuclei in this ectoderm “ being rather iri-egnlarly 
arranged, the pulmonai-y ingrowths [i.e. the furrows] forc- 
ing their way between them.” dlie more distal gill-lamellm 
(by which the author means the se])ta) are the oldest, as in 
Ijimulus (p. 217). Simmons’ paper is dealt with again 
further ou (p. db). 
Comparison with the gill-books of Limulus. — It would be 
profitable here to institute a comparison with the gill-books 
of L i in u 1 u s. 
According to the description and the figures of Kingsley 
(’85), the gill-leaves of the American Limulus arise as out- 
growing folds of the epithelium of the posterior side of the 
apjiendages, their formation being accompanied by a slight 
in-tucking of the epithelium between them, and taking place 
in the same order as the pulmonary saccules in spiders. The 
epithelial walls of each outwardly directed fold are, however, 
not in contact along their ba.sal surfaces, and have apparently 
not been suddenly reduced in thickness, thus differing in 
these two points from the rudimentary lung-books. 
In the Japanese Limulus the process appears to be some- 
what different. According to Kishinouye (’91), the proximal 
jiortion of the appendage is much thicker than the distal 
jiortion and is ])rovided with many transverse furrows or 
invaginations, the tissue between two furrows giving rise to a 
gill-lamella. At any rate both forms agree in one main 
