liESriKATOKY OKGANS IN AUANE/E. 
27 
Li m ulus. We should have iio difficulty in imagiuing a case 
in which the cells divided so as to cause the folds to expand 
simultaneously in both directions, and the result would be a 
structure intermediate between the gill and the lung-book. 
The foregoing paragraphs lead up naturally to the simple 
and ingenious hypothesis first put forward by Kingsley (’85) 
to explain the derivation of the lung-books from gdl-books 
(see Kingsley’s explanatory figs. 18-20). He simply assumes 
that simultaneously with the sinking of the whole organ the 
inwardly directed folds of the gill-books became exaggerated, 
while those directed outwai'ds correspondingly decreased. In 
this way an intermediate type of respiratory organ would 
first be obtained, representing the condition in tlie animal 
when it was leaving the water and seeking a terrestrial life. 
Finall}-, the lung-book type would be reached by the com- 
plete suppression of any tendency of the folds to grow 
outwards. 
Now, from a morphological point of view there should be 
no difficulty in accepting this hypothesis. The passage from 
a gill-lamella with three free outer edges to a lung-septum 
with only one such edge is perfectly simple and easy to 
imagine. It now really constitutes the only assumption not 
directly proved ontogenetically which we have to make in 
deriving the Arachnid lung-book from a Limuliue gill-book. 
For the two remaining conditions necessary for such an 
origin, namely, the appearance of the oldest septa on the free 
posterior side of the appendage and the subsequent subsi- 
dence of the latter, are observed embryological facts. To 
return to the first point, the ontogeny, although it does not 
exactly prove it, furnishes us, nevertheless, with some evi- 
dence which tends to show that the folds were originally 
designed to grow outwards and not inwards. For, so far as 
1 could make out, the two walls of the most distal se 2 )tum or 
outwardly directed fold are formed simultaneously, and 
followed later by the simultaneous apjiearance of the two 
walls of a second fold also dii-ected outwards, and so on (see 
fig. 11). It cannot be denied that each such fold, on its first 
