540 PROFESSOR E. RAY LANKESTER, 
IV. The common characters of the lamelligerous appendages 
of Scorpio and Limulus.—When we have once, on the ground 
of a certain general agreement in structure and of a definite 
identity in relation to other parts which correspond one to 
another, started the hypothesis that the lamelligerous 
appendages of the Scorpion agree each to each in their order 
with the lamelligerous appendages of the King Crab, two 
further proceedings are naturally the consequence. We 
inquire first of all whether there are any less obvious agree- 
ments in the structure of the organs compared which may be 
brought out and made to give their testimony in favour of 
our hypothesis, and, secondly, we inquire how can we form 
a plausible conception of the origin of the two sets of struc- 
tures from one set of organs present in a common ancestor 
of Limulus and Scorpio? this last inquiry having especial 
value, in that it may lead us to give due value to structures 
present either in Scorpio or Limulus which had appeared 
previously to have no special significance in the matter. 
A close comparison! of the lamelligerous appendages of 
Scorpio and Limulus—including under this head the 
pectines and the pulmonary books of the former, anu the 
branchial books of the latter—reveals the important fact that 
they agree closely with one another in the mode in which 
the lamelle are set upon the supporting axis 
In all, we find an axis springing from the body wall, 
transverse to which, on its posterior face, ave set 4 series of 
lamelle. In order to compare one of these «ppendages 
with another, it is necessary that all should be p.aved in one 
and the same position. We must be careful not to compare 
the anterior aspect of one with the posterior aspect of the 
other. In the woodcuts, figs. 6 and 7, the posterior face of 
the appendage as it hangs from its sternal attachment has 
been represented. 
There is no difficulty about determining this face for the 
pectines of the Scorpion or for the branchial appendages of 
Limulus, but the pulmonary books of the Scorpion require 
some consideration. Supposing them to have once been 
external, we must suppose that, with the gradual invagina- 
1 The account which I give in the text of the lung-books of Scorpio 
differs a good deal from that which is current, due to Joh. Muller as long 
ago as 1828. I have not had specimens sufliciently well preserved to enable 
me to determine the relation and possible adhesions of the proper wall of 
the pulmonary sac (the invaginated sternal surface) to the lamellse, but have 
freed the appendage from the investing membrane. I hope to be able by 
the examination of fresh specimens to give on a future occasion a more 
thorough account of the pulmonary sacs and lamelligerous appendages of 
the Scorpion. 
