364 PEOF. E. E. LANKESTEE 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 lAmulus, 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 i-emarkable 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 tliat which it originally occupied. This interpretation of the entochon- 

 drites is favoured by the fact that the small mesosomatic entochondrites oi Limulus (and 

 the single mesosomatic cntochondrite of Scorpio) are in close relation to the sternal epi- 

 dermis and lie beneath the nerve-cords, altliough 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 



