PART II. 



SYSTEMATIC CLASSIFICATION OF ECHINI. 



As urged by Hyatt, a natural classification of organisms should be based on a comparative 

 study of the young and adult, fossil and living forms; the young because through stages in 

 development they throw light on ancestry; the adult because such are the full expression of 

 generic and specific characters; the fossil because as an essential part of a group they cannot 

 be ignored, and also because to the past one must look for the ancestors of the living. 

 The living representatives as an essential part of a group need consideration, and if 

 one's work is largely palaeontological, a study of the living is of first importance, as the study 

 of soft parts throws great light on the structure of hard parts, as I showed (1890) in studies 

 of Pelecypoda. Variation is only second to development as a basis for studying genetic rela- 

 tions, as variants, both arrested and progressive, have a direct relation to the characters of 

 more primitive or more specialized species or genera in a systematic series. 



Echini are classified almost entirely on the structure of skeletal parts, so that well preserved 

 fossils can be studied with almost as much accuracy as living forms. Mortensen has urged 

 the importance of pedicellariae and also spicules in the tube-feet as a basis of classification. I 

 believe no one has shown that these structures have a differential value in ontogeny or in evolu- 

 tionary series. Moreover, they are minute and diflftcult structures to ascertain. If one 

 attempts to follow Mortensen's splitting of genera on these characters, it seems that it is neces- 

 sary to be a specialist on these particular parts in order to perceive the fine distinctions that 

 he draws. Every part of an organism is surely worthy of careful study ; but to base classifica- 

 tion on such minutiae of no known evolutionary value is undesirable. 



The classification here offered conforms for the most part with what I gave in 1896, differing 

 from it only in minor details and going further in certain groups. It is based essentially on 

 the structure of the adult and the development of the same. While no single character has been 

 followed, the characters taken into consideration are: the ambulacrum, interambulacrum, 

 coronal imbrication, basicoronal plates, ventral resorption of corona, ocular and genital plates, 

 periproct, peristome, Aristotle's lantern, perignathic girdle, spines and tubercles, gills and 

 sphaeridia. The relative value of these parts naturally differs in different groups of the Echini. 

 Of Palaeozoic Echini all the genera, except those imperfectly known, are here briefly con- 

 sidered, but of post-Palaeozoic forms only the larger groups are taken into account. 



It may be said with truth that all Echini belong distinctly to this class and make no close 



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