THE PHYLUM ECHINODERMATA 



as an extreme modification of the bilateral-triploblastic-eucoelomate plan 

 which appears in all the more highly developed phyla — a modification which 

 involved the remodelling of typical bilateral organization along the lines 

 of radial symmetry. The larvae (bipinnaria, pluteus, auricularia, etc.) char- 

 acteristic of the various classes show many similarities, and it has been 

 suggested that they may all have arisen from a hypothetical ancestral larval 

 form called the dipleurula. 



But how does it happen that an animal which is radially symmetrical as an 

 adult has a bilateral larva? Two answers mav be considered. Either this 

 larval stage has developed secondarily and represents what happened to be 

 produced in the adjustments of larval life during the long evolutionary 

 history of the echinoderms; or, more probably, it occurs in the development 

 of echinoderms because it represents a stage in their ancestry, like the 

 fish-like stages in the ontogeny of a higher vertebrate. If we regard a 

 dipleurula-like animal as representing a bilaterally symmetrical ancestor of 

 echinoderms, we may suppose that this ancestor became attached and 

 acquired a five-part radial symmetry in correlation with a sessile way of life. 

 The oldest known echinoderm fossils represent attached forms, and the fixed 

 habit has been retained by modern crinoid sea lilies. The ancestors of 

 asteroids, ophiuroids, holothurians, and echinoids, on the other hand, no 

 longer attached forms, have become variously modified for free life. But even 

 the free-moving asteroids and feather stars show a temporary attached phase 

 at the time in the life cycle when the bilaterality of the larva is replaced 

 by the radial symmetry of the adult. Finally, it should be noted that in 

 some holothurians and sea urchins, the secondary radial symmetry has begun 

 to be replaced by a new bilaterality, which is unrelated to the bilateral 

 svmmetrv of their larvae. 



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