18 NOMENCLATURE. 



limits we assign to these categories in some way readily understood ; and 

 this the individuals or groups of individuals themselves belonging to the 

 different categories will supply. Taking a small group like the Echini, in 

 which the number of species is not large, we can readily follow in all the 

 species the value of the characters which have been called ordinal, generic, 

 or specific, and cannot fail to see how inadequate our diagnostic descriptions 

 become as soon as we attempt to incorporate with them even the scanty 

 information of the present day of the life history of any one species. 



The genera recognized are usually based upon some structural Features 

 derived from the pedicellariae, the poriferous zone, the character of the tuber- 

 cles or their arrangement, the abactinal or actinal system, and general facies. 

 Their value, when tested by our present knowledge of the changes they 

 undergo, seems limited almost to convenient headings or keys for the more 

 ready identification of species. Genera, as we recognize them among Echini, 

 are certainly not founded upon features of general and permanent value, but, 

 on the contrary, upon features applying only to a few species, and of very 

 limited application. During the growth of the Sea-urchins of different 

 groups, certain parts (different for the various divisions) change rapidly, 

 others do not. and it is to these permanent characters, limited in their appli- 

 cation, that we must resort as guides for our generic tests. 



The structure of the pedicellariae is very variable; in the same species 

 they may be present in abundance, or totally wanting in specimens living 

 under the same conditions. Yet. a- characteristic of some groups, they fur- 

 nish, as far as we know, excellent points of distinction. In some genera the 

 number of the rows of vertical tubercles is constantly increasing; in others, 

 the number of the tubercles alone increases. The perforation and crenula- 

 tion of the tubercles, as far as we can judge from recent species, furnish excel- 

 lent characters for a class of subdivisions; but what shall we call them? 

 The poriferous zones give us good features to distinguish such groups in 

 the adults as Toxopneustes, Toxocidaris, Loxechinus, Sphaerechinus, yet 

 these same characters would not help us to place the young of the above 

 in their proper genera; though, knowing the limits of the changes to which 

 the poriferous zones are subject, they arc an invaluable auxiliary in classifica- 

 tion, in spite of their uniformity among all the Cidaridae. 



The general facies which might, with accuracy, tell us that we deal with 

 a young Echinometra, would certainly mislead us when we have before us 

 the young of many a Spatangoid or Clypeastroid. The notches of the actinal 



