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phylogenetic characters than they. This is particularly true of the am- 
bulacra and therefore when the characters of the ambulacra and those 
of the pedicellariae disagree, we should follow the former. His second 
argument is that neither Mortensen nor any other writer has shown, 
or apparently can show, that pedicellariae have any phylogenetic signi- 
ficance. They seem to pass them no stages of development but are the 
same in very young individuals as in adults. Indeed Mortensen him- 
self points this out as one of the reasons why they are so useful in 
distinguishing species. Jackson argues, and with absolute correct- 
ness it seems to me, that characters which in themselves show no stages 
of development are of little use in tracing the phylogenetic history of a 
group. | 
Both Döderlein and Mortensen labor underthe misapprehension 
that Jackson’s and my rejection of Mortensen’s classification is 
based on the “inconvenience” (because of their microscopic character) 
and variability of the pedicellariae and spicules. It is true we have 
each referred to these obvious objections but neither of us has suggested 
them as fundamental. In several papers, I have laid considerable em- 
phasis on the variability of the pedicellariae because Mortensen gives 
the impression in his writings that they are less variable than the cha- 
racters of the test, and I am convinced that this is not the case. But 
I certainly do not claim, as Döderlein seems to think I do, that the 
characters of the ambulacra and other parts of the test are not also 
variable. Unfortunately all characters in living organisms are more or 
less variable. 
In conclusion, let me seem up thus. I reject Mortensen’s classi- 
fication and accept Jackson’s because: | 
1) the characters shown by the test, Ibelieve to be older 
and of more fundamental importance than are those shown 
by any of its appendages; 
2) the characters of the test and “lantern” show distinct 
stages in development in the ontogeny of the individual, 
which stages I believe to be of definite phylogenetic signi- 
ficance and of fundamental importance; 
3) the pedicellariae show no evident stages of develop- 
ment and hence seem to have no phylogenetic significance. 
