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THE AMERICAN NATURALIST [Vol. XLVII 



cialized, sometimes specialized far beyond what is seen 

 in any other member of the group, while others will be 

 in a very rudimentary or primitive state of development, 

 or even absent altogether. 



The echinoderms differ very abruptly from the crusta- 

 cean line of descent from which they took their origin 

 and, similarly, each echinoderm class differs very ab- 

 ruptly from all the others. We see in all the echinoderms 

 to-day most perplexing combinations of primitive and 

 highly specialized characters associated in all sorts of 

 ways, and this leads us naturally to the assumption that 

 there was no definite intergrade between the echinoderms 

 and the barnacles, but that the former sprang from the 

 latter or, more strictly speaking, from the same phylo- 

 genetic line which may be traced by easy stages to the 

 latter, by a broad saltation in which the assumption of 

 the free habit and the correlated assumption of pentara- 

 diate symmetry combined to make the existence of inter- 

 grading forms impossible, while at the same time it 

 resulted in the formation by the echinoderms, at the very 

 moment of their origin, of two widely diverse stocks, the 

 heteroradiate (including the Pelmatozoa, the Echinoidea 

 and the Holothuroidea) and the astroradiate (including 

 the Asteroidea and the Ophiuroidea) between which there 

 are, and can be, no intergrading forms. 



Thus in dealing with the echinoderms we must be ever 

 on the alert to detect sudden saltations. We must also be 

 prepared to eliminate from our minds all ideas of hypo- 

 thetical ancestors from which all echinoderms are com- 

 monly supposed to have been derived, but which probably 

 never existed ; and, along with the hypothetical ancestor 

 myth, to banish from our thoughts all ideas of funda- 

 mental echinodermal structures, equally non-existent. 

 No echinodermal structure is of such fundamental im- 

 portance in the economy of the animals that it can not be 

 either profoundly modified or even dispensed with 

 altogether under special conditions, reverting to a type 

 more or less characteristic of some other phylum. The 



