A. H. Clark — Pentamerous Symmetry of Crinoidea. 357 



brought to their present state of perfection, or were even more 

 highly specialized, in the earliest geologic epochs of which we 

 have any record. The enormous time through which they 

 have passed without any appreciable change toward further 

 specialization has resulted, by the operation of the law of accel- 

 eration of development, in pushing the various ontogenetic 

 steps further and further back until all such stages as the 

 biserial arrangement of the crinoid arm, the change from 

 bilateral to pentamerous symmetry, the reduction and transfor- 

 mation of the ambulacral plates in the crinoids into division 

 series, etc. have been entirely lost. The echinoderms as a 

 whole I believe can be compared with the other invertebrate 

 groups in their mode of development only in the way that the 

 curiously specialized Gecarcinus can be compared with other 

 crabs, or Hylodes with other frogs ; in other words, they form 

 a group in which extreme specialization and its effect upon the 

 earlier stages of the ontogeny has obscured or obliterated 

 much of the phylogenetic data ordinarily available. 



Larval echinoderms are as extraordinarily specialized as the 

 adults, but along radically different lines ; many of them have 

 specially developed skeletons, entirely different from those 

 of the adults, which are lost in later life. It seems to me that 

 the echinoderm larvae, owing to their extraordinary speciali- 

 zation, must be treated almost as a different class of animals 

 from the adults, specialized along entirely different lines, and 

 fitted for an entirely different mode of life ; and that we can 

 expect to learn little more about the interrelations of the 

 echinoderm groups from the study of their larvae than we 

 could of the relationships of the groups of lepidoptera, diptera 

 or coleoptera from a study of their caterpillars or maggots. 



Am. Jour. Sci.— Foueth Series, Vol. XXIX, No. 172.— April, 1910. 

 24 



