ENDOSKELETAL SYSTEMS OF LIMULUS AND SCORPIO. 365 
they had converged towards the median line in the mesosoma (as we sce is the case in 
representatives of allied groups, such as Serpula), then there would be no difficulty in 
accounting for the present position of the nerve-cords in relation to the entochondrites 
by supposing that they tended subsequently to the detachment of the prosomatic 
entochondrite to take up a position more and more coincident with the median ventral 
line, although in the mesosoma they had already taken up such a position. Accordingly 
the nerve-cords in the prosoma would be able to take up their present position beneath 
(7. e. ventrad of) the prosomatic entochondrite, whilst in the mesosomatic region the 
nerve-cords, already occupying a median position, would necessarily remain superior 
(7. e. dorsad to) the mesosomatic entochondrites. Such movements of masses of tissue as 
are here postulated are entirely in accordance with well-established conclusions. There 
is no doubt that the double nerve-cord of Arthropods and Chetopods owes its double 
character to the fact that it originated as two widely separated lateral tracts of nervous 
tissue, which have gradually (in the course of ancestral development) converged towards 
the middle line, as is also the case in the independent phyla of the Leeches and the 
Molluscs. 
There is also no doubt that these nerve-cords originated in the epidermis, and that 
in some animals they still remain actually as thickened ridges of that layer, whilst in a 
large majority they have become detached from that epidermal connexion (although 
maintaining it in their embryology), and have sunk inwards through connective tissue 
and muscle until they lie well within the body. There is no animal in which this 
detachment of the nerve-cord from its primitive relation to the epidermis is carried so far 
as the Scorpion, where, as seen in the sections drawn in Pl. LXXXI. figs. 1, 2, um, the 
nerve-cord attains in the mesosoma almost an axial position. Just as the mass of tissue 
called nerve-cord can move from its primitive relations, so, it appears reasonable to 
admit, can other tissue-masses, and accordingly amongst others the dense subepidermal 
entochondrites. Possibly the application of this principle of the in-sinking of primi- 
tively subepidermal skeletal tissue may throw some light upon the skeletal structures 
of Vertebrata. At any rate it is a legitimate hypothesis in regard to the entochondrites 
of Arthropoda, and enables us to understand the nature of these bodies and the muscles 
attached to them. The muscles attached to the entochondrites are primarily the 
muscles attached to the midsternal region of the segments in which such entochon- 
drites occur. This is.obvious enough with regard to the small mesosomatic entochon- 
drites of Zimulus, and it will be found to give an intelligible explanation of the muscles 
attached to the great prosomatic entochondrite. 
It is to be noted that the inter-entapophysial ligaments which run on each side, right 
and left, along the dorsal surface of Limulus, passing from one entapophysis to the next, 
are of similar nature and origin to the entochondrites. They represent a tract of 
detached subepidermal connective tissue belonging to the tergites, just as the ento- 
chondrites represent subepidermal connective tissue of the sternites. 
