LIMULUS AN ARACHNID. 543 
ter of this axis is obviously an adaptation to the branchial 
function of the lamella combined with a locomotor 
function. 
The lung-book of Scorpio has no locomotor function, and 
it is protected by the recess of the sternum, in which it lies, 
It is not tactile, nor is it exposed to desiccation and rough 
usage, as are the pectines. It is specialised for respiratory 
purposes. The axis is exceedingly small and simple, for 
the greater part of its length adherent to the invaginated 
sternal wall, leaving, however, a small free distal portion 
(see Pl. XXVIII, fig. 2a). Its walls are quite free frem 
chitinisation, and of great delicacy. It is little else than a 
horizontal vascular tube supporting the lamelliform bags 
into which its cavity leads (Pl. XXVIII, 24a, d). 
Though the axis is here reduced to its simplest ex- 
pression, it is not possible to overlook init the representative 
' the vertically compressed chitinised axis of the pecten, 
anc © the horizontally compressed chitinised axis of the 
’. Hypothesis as to the mode of origin of the three varieties 
ol lameiligerous appendages in Scorpio and Limulus.—The 
view which I have advanced in this memoir as to the prac- 
(ical identity of the gill-books of Limulus and the lung- 
oooks of Scorpio implicitly contains the affirmation that either 
the structures of Limulus have been derived from those of 
Scorpio, or those of Scorpio from those of Limulus, or that 
a third (now extinct) form has given rise to both Limulus 
and Scorpio. Further, it is to be observed that such 
extinct form might be more like to Limulus than to 
Scorpio, or vice versdé, in respect of any particular element of 
structure. 
To make a long story as short as possible I may say that, 
without prejudicing the recognition of the (as I think) well- 
established morphological identities above pointed out, we 
may best explain their existence by assuming that an aquatic 
form breathing the dissolved oxygen of the water inhabited 
by it, by means of book-like gills, was the common ancestor 
of Limulus and of Scorpio. From the book-like gills of this 
ancestral form the broad series of Limulus and the narrower 
lung-books, as well as the pectines or combs of the Scorpion, 
have been derived. The form of the book-like gills of this 
Arachnidan ancestor was probably something intermediate 
between the three existent modifications of it, and best con- 
ceived of, perhaps, by imagining the teeth of the Scorpion’s 
“‘ pectinate organs” to become soft and flattened and 
increased in number (see Pl, XXIX, fig. 1). 
