478 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 ccelomic lining is partly enteroccelic 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 ccelomic 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 
Polycheta, we still see it stated that the musculature, both circular 
and longitudinal, arises from the ectoderm. In the Oligocheta 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 
ccclomic 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 
