ENDOSKELETAL SYSTEMS OF LIMTJLUS AXD SCORPIO. 365 



they had converged towards the median line in the mesosoma (as we see is the case in 

 representatives of allied groups, such as Serjjula), 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 

 (i. e. ventrad of) the prosomatic entochondrite, whilst in the mesosomatic region the 

 nerve-cords, already occupying a median position, would necessarily remain superior 

 (i. 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 Chaetopods 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 conuexion (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 PI. LXXXI. figs. 1, 2, nn, 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 Vertebrate. 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 Limulus, 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-en tapophysial 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. 



