47 8 THE ORIGIN OF VERTEBRATES 



medullate at the same time as those in the skin, and both obtain 

 their medullary sheaths earlier than any other nerves, whether 

 afferent or efferent. However difficult it may be to explain this fact, 

 only one conclusion seems to me possible — these Pacinian bodies, like 

 the skin Pacinians, originate from a nest of surface epithelial cells, a 

 conclusion which is extremely probable on my theory of the origin of 

 vertebrates, but not, as far as I can see, on any other. 



At the present moment the weight of evidence is, to my mind, 

 in favour of the lining endothelium of the ccelomic cavities being- 

 composed of free cells, unconnected with the nervous system rather 

 than the reverse, but I must confess that the question is undecided. 

 If it be true that the coelomic lining is partly enterocoelic and partly 

 gonoccelic, as Lankester teaches, then it would be natural that its 

 cells should be in connection with the nervous system, to some 

 extent at all events. This view is, however, based on very slender 

 foundations. If the mesothelium is composed of cells capable of 

 becoming free, it cannot give rise to the skeletal muscles, and it 

 cannot therefore be right to speak of the skeletal muscles as 

 derived from the lining cells of a part of the primary ccelom. 

 The phylogenetic history of the musculature of the different 

 animals points strongly to its intimate connection with and deriva- 

 tion from surface epithelial cells rather than from coelomic mesothelial 

 cells. Thus in the ccelenterates, as seen in Hydra, the muscular 

 layer arises directly from a modification of the surface epithelial 

 cells ; and right up to the annelids, even to the highest form in the 

 Polychaita, we still see it stated that the musculature, both circular 

 and longitudinal, arises from the ectoderm. In the Oligochseta and 

 Hirudinea, according to Bergh, there are five rows of teloblasts on 

 each side, of which four are ectodermic and give rise to the nerve- 

 ganglia and the circular muscles, while one is mesoblastic and forms 

 the nephridial organs and the longitudinal muscles. (The latter 

 statement is, according to Bergh, well known, and is not particularly 

 shown by him. These longitudinal muscle-bands always lie close 

 against the nervous system at their first formation, and may well 

 have been derived in connection with it.) 



It is apparently only in the Vertebrata that the lining cells of the 

 cojlomic cavity are definitely stated to give origin to the body-muscu- 

 lature, and taking into account on the one hand the evidence of 

 Graham Kerr as to the intimate connection between nerve-cell and 



