INTRODUCTION. 



The principles made use of and the methods of work employed in studying Echini are here 

 treated before starting into the detailed considerations. The one essential principle that has 

 been applied consistently throughout is the principle of stages in development and the constant 

 comparison of these stages with the characters of more or less closely associated types. This 

 principle applied conjointly to fossil and living forms was urged by Professor Louis Agassiz, 

 and was the center of a large part of Professor Hyatt's work. An attempt has been made to 

 bear in mind the characters of the young, adult, and old age, and to compare these with the 

 young or with the adult of simpler or more specialized types, both living and fossil. As far as 

 material permitted, the entirety of the organism has been taken into account. If Echini have 

 taught me anything, they have impressed upon me that classification should be based on the 

 sum of the characters and not on single characters. 



Herbert Spencer pointed out the distinction that growth is mere increase in size without 

 the addition of differential characters; development is the addition of differential characters. 

 Development proceeds rapidly during the early growth of an organism, but in later life we may 

 get extensive growth without any development. In Palaeozoic Echini, development, that is, 

 the addition of new characters, is often long continued, so that new characters, especially 

 additional columns of interambulacral plates, may be taken on up to or even dorsal to the 

 mid-zone. Then in the dorsal region we come into the zones of localized stages in development, 

 where the young, last added plates are progressively taking on their full characters as they are 

 pushed ventrally by the intercalation of still younger plates on their dorsal border, between 

 them and the apical disc. Dorsally, we also often find senescent stages or characters of regres- 

 sive development, especially marked by the dropping out of columns of interambulacral plates, 

 as in Melonechinus indianensis (Plate 53, fig. 1). 



In the Palaeozoic Echini, as elsewhere, specimens of a species vary in size, and doubtless 

 this usually means age, but really young specimens are almost unknown. The only very young 

 one seen is Lovenechinus missouriensis (Plate 39, fig. 1; compare with adult, Plate 39, fig. 5). 

 In the absence of the young, youthful stages are gathered from the characters of the plates 

 at the ventral area of the corona, which, as shown in my earlier paper, is an area that preserves 

 the early stages of development in a remarkable degree. Except when destroyed by resorption, 

 this area in a perfect specimen represents the first interambulacral plates built in any corona, 

 and the first ambulacral as well, unless they have passed on to the buccal membrane. Wliile 

 these ventral plates have increased in size since early youth, their presence, number, angles, 



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