122 GEOEGE W. FIELD. 



long arms, and particularly by modification of the external form 

 by changes in the direction of the ciliated bands, as pointed 

 out by Johannes Miillerj into the forms characteristic for the 

 various Echinoderm groups. In their ontogeny the Auricularia 

 and the Bipinnaria have travelled together for some distance, 

 as shown by the fact lately pointed out by Semon (38) that 

 the Bipinnaria passes through an Auricularia stage. We may 

 suppose that the bilateral form after a period of free-swim- 

 ming life became sedentary, and after this the bilateral 

 symmetry became more or less disguised by a radial sym- 

 metry. From this sedentary form, the Pentactea of Semon, 

 the ancestors of the present Echinoderm groups have been 

 derived. The earliest arising were the Synaptidee through 

 some archaic form ; next came off the ancestors of the Holo- 

 thurians, and later the ancestors of the Crinoids, and latest 

 the ancestor of the Echinids, Ophiurids, and Asterids. We 

 are certainly justified in applying to the tentative theory of 

 Echinoderm phylogeny the principles which are accepted in 

 attempts to trace the phylogeny of other classes, namely, that 

 it is not to be expected that many of the ancestral forms con- 

 necting the groups are to-day accessible for study, either alive 

 or as fossils ; and in view of the failure of the numerous and 

 carefully formulated theories of Echinoderm descent it seems 

 necessary to believe that the various groups were derived from 

 one another only through intermediate forms between each 

 group, which forms, however, probably persisted but a com- 

 paratively short time, and palseontological evidence of them is 

 very scanty, or in the vast majority of cases entirely wanting. 

 The corresponding stages, too, have been for the most part 

 eliminated from the ontogeny. 



The groups of the Echinids, Ophiurids, and Asterids, and a 

 part of the Holothurids have been coenogenetically modified 

 for a creeping life, the original excretory system assuming 

 the locomotor in addition to the earlier acquired sensory and 

 respiratory functions. The early appearance of radial sym- 

 metry in the free-swimming larva shown in the radial out- 

 pushings of the hydrocoel wall at that stage of the ontogeny 



