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J. S. KINGSLEY. 
organs in the two groups this physiological change is too great 
to be readily accepted until we know more about the develop- 
ment of these organs in the Arachnids. Still, the description of 
Metschnikolf quoted above is, so far as it goes, not wholly in- 
compatible with this view, that the primary stigma formed by 
the insinking of the respiratory book is not the functional one 
of the adult, since this author notices its decrease in size, while 
the mode of origin of the lamellae from the dorsal surface of 
the cavity is still more in its favor. Emerton (’ 72 , pi. 2, figs. 
11, 13, 15) represents the abdominal appendages of the embryo 
Pholcus as broad and like those of Limulus at a corresponding 
stage, a fact opposed to their being merely modified ambula- 
tory appendages, but in full accord with their homology with 
those of Limulus. 
On the other hand, the derivation might be much more 
direct, and thus avoid the inversions and the functional 
changes. As I have mentioned above, the process of formation 
of the gill-leaves is largely by a process of outgrowth, but there 
is also a slight ingrowth, especially noticeable at the distal 
portion of the appendage. This, however, disappears with 
growth, but is very noticeable in all my sections. To transform 
the gill of Limulus into the lung of Scorpio it is only necessary 
that, together with the sinking of the whole organ, as described 
above, the inpushings of the integument to form the lamellae 
should be exaggerated, and the outgrowths correspondingly 
decreased. On Plate XXXVII, figs. 18 to 20, 1 have diagram- 
matically illustrated the steps in the process, the gill-leaves 
being few in number to secure clearness. In 18 we have the 
typical condition found in Limulus, one appendage being shown 
half in section and half in perspective. In 19 we have an in- 
termediate condition, when, as suggested above, the animal 
was leaving the water and seeking a terrestrial life. Here the 
gill-bearing appendage ( ga .) is partially sunk in the surround- 
ing tissues to secure protection. The same causes would also 
tend to produce a similar change in the gill-leaves ( gl .), and 
they would also tend to be formed rather as ingrowths than as 
protruding processes. This change in structure would be the 
