364 PROF. E. R. LANKESTER ON THE MUSCULAR AND 
The hollow in-sinkings of the chitinous surface of the body, in connexion with the 
attachments of muscles so largely developed in Limulus, form, it may be noted, an 
Arachnidan character. Such cupping of the chitinous integument is seen in the 
Arachnid Thelyphonus. The remarkable hollow tendons of Limulus originating as 
stigmata at the bases of the mesosomatic appendages, and giving attachment to the 
great thoraco-branchial muscles, are of the same nature as the dorsal entapophyses. 
These oblique or vertico-oblique muscles of Limulus (the thoraco-branchials) consti- 
tute the chief difference between the musculature of Limulus and Scorpio. ‘They are 
unrepresented in Scorpio; they cannot be derived from any muscles existing in that 
animal or in the hypothetical common ancestor of the Arachnids. ‘They must be 
regarded as new structures, special to the Limuloid modification of the type. 
It may be laid down as a guiding principle in the study of phylogeny or the genea- 
logies of animals and plants, that organs do not arise de novo, and that apparently. new 
organs are to be traced to pre-existing organs, by the modification (division, expansion, 
atrophy, or other change) of which they have gradually been brought to their present 
condition. It is questionable, however, whether this principle can be applied to the 
phylogeny of muscles. Muscular tissue apparently may replace, and does actually 
replace, ordinary fibrous or other connective tissue, and thus a muscle may be formed 
where no muscle previously existed. The development of striped muscular tissue in 
Limulus is exuberant in a remarkable degree, and it is by no means an unwarrantable 
assumption that in this and in other Arthropods new muscular connexions are brought 
about by the gradual substitution of muscular for connective tissue. This of course 
merely implies that muscular tissue, like connective tissue, fine blood-vessels, and 
nerves, is not liable to restriction in the direction and manner of its growth in strict 
accordance with the segmentation impressed upon an animal in early stages of its 
genealogical history and inherited in a more or less perfect form at the present day. 
Nature of the Entochondrites.—I have come to the conclusion that the prosomatic 
and smaller posterior entochondrites, both of Limulus and Scorpio, are, in so far as 
their “ body” or central part is concerned, merely the original subepidermic connective 
tissue of the sternal surface of the segments in which they occur, which has become 
thickened and cartilaginoid, and has at the same time floated off, as it were, from the 
sternal surface and taken up a position deeper, that is to say nearer the axis of the 
animal, than that which it originally occupied. This interpretation of the entochon- 
drites is favoured by the fact that the small mesosomatic entochondrites of Jimulus (and 
the single mesosomatic entochondrite of Scorpio) are in close relation to the sternal epi- 
dermis and lie beneath the nerve-cords, although the large prosomatic entochondrites of 
both Limulus and Scorpio have the nerve-cords below them. Supposing the detachment 
from the sternal integument of the mass of connective tissue forming the prosomatic 
entochondrite to have occurred at a period when the nerve-cords were still quite lateral 
in position (as they remain to this day in Peripatus) in the prosomatic region, although 
