536 
J. S. KINGSLEY. 
this respect from the corresponding embryonic limbs of 
Arachnids. 
The operculum never develops gills, but in the adult bears the 
genital openings. In the others the leaflike respiratory organs 
first appear at the stage of fig. 14. The method is shown in 
figs. 37 and 38, and needs but little description. The leaves of 
the gill book arise as outgrowths from the posterior surface of 
the appendage, accompanied apparently by an intucking of the 
adjacent epiblast. This operation takes place first at the distal 
portion of the appendage, and new leaves are added at the base, 
the whole series overlapping each other like the shingles on a 
roof. 
So far as I am aware, Professor Van Beneden was the first 
to suggest ('71) the homology between the branchiae of Limulus 
and the pulmonary books of the Arachnids. This was further 
elaborated by Professor Lankester (’81). At first this resem- 
blance seemed as far-fetched to me as it did to Dr. Packard 
(’82, p. 290), but subsequent studies seem to me to indicate its 
general validity, although I am not ready to follow all of 
Professor Lankester’s intermediate steps, nor those of 
McLeod (’82). 
Of the development of the pulmonary organs in the Arachnids 
the literature is extremely scanty, but with Lankester I am 
inclined to believe that when more is known of it, it will be 
found that the lamellae arise in connection with the temporary 
abdominal appendages. On this point Metschnikoff, treating 
of the Scorpion, says (’71, p. 225): “ The lungs also arise as 
invaginations of the epiblast (Hornblattes), which appear close 
under the segmental appendages of the four abdominal seg- 
ments (Taf. xvi, fig. 12, pn). 1 They appear from the first as 
pocket-like sacs, which open by a broad mouth. With the 
further development of the lung sacs, which is accompanied 
by an atrophy of the abdominal segments [? appendages] (with 
the exception of the second pair of the same), they become 
more spacious and deeper. Only at the latest embryonic stage 
(Taf. xvi, fig. 14 from the ventral, fig. 15 from the dorsal, sur- 
1 Reproduced on PI. XXXVII, fig. 15. 
