340 
PROFESSOR E. RAY LANKESTER. 
up my over-strained hypothesis. I trust that the failure of my 
previous suggestion will not unduly prejudice those interested 
in this subject against that which I now advance. 
Since my memoir f Limulus an Arachnid/ Dr. MacLeod, 
of Brussels, has published some speculations on this subject, in 
which he puts forward an ingenious theory of his own as to 
the mode in which the lamelligerous appendage of a Limulus- 
like animal might be converted into the lamelligerous lung- 
book of an Arachnid. I will not enter into a discussion of Dr. 
MacLeod's hypothesis, but will merely point out that inasmuch 
as it deals with not the less modified lung-book of Scorpion, 
but the more modified lung-book of the Araneina, it is unsatis- 
factorily elaborated. The lung-book of Scorpio has a definite 
axis carrying the leaf-like lamellae, and corresponding to the 
axis of the same animal's pecten. Such an axis is not present 
in the Araneine lung-book, and yet must be accounted for as 
a primary structure in any theory as to the origin of these 
organs. 
The hypothesis which I now put forward is perfectly simple, 
and leaves, I think, nothing to be desired. In Limulus, as 
in Scorpio, there is on each side of the sternal surface a great 
blood-sinus in free communication with the lamelligerous 
organs. Let us suppose such to have been the case in the 
common ancestor of these two animals, and let us suppose that 
this ancestor possessed six pairs of mesosomatic appendages, of 
which five were lamelligerous and intermediate in form between 
the pectens of Scorpion and the recent Limulus appendage. 
Now, suppose that in the Scorpion branch of the family the 
mesosomatic appendages grew relatively smaller and smaller, 
were no longer locomotor organs, but purely respiratory, and 
served for aerial rather than aquatic respiration. If we ima- 
gine the four hinder pairs of these reduced appendages to have 
taken on in the embryonic condition a very common 
trick of growth, viz. an inward growth of invagination, so that 
they grew into the Scorpion’s body, turning their outside in, 
just as a glove may have all its fingers and part of the hand 
turned outside in — then we should have without further alte- 
