GROWTH-CHANGES IN BRITTLE-STARS. 



By Hubert Lyman Clark. 



INTRODUCTION. 



Although the systematic work of Ljungman, Liitken, Lyman, and 

 Koehler, which has brought to Hght hundreds of species of brittle-stars, has 

 been of a very high quality, the real interrelationships of the genera and 

 families within the group are still virtually unknown. Indeed the defini- 

 tions of most of the families and of many of the genera are so hazy as to be 

 exasperating, and the more one studies the system at present in use the more 

 assured one becomes that it is artificial and unnatural. The first step 

 towards a rational classification was that taken by Bell (1892), when he 

 divided Recent ophiurans into three orders, based on the character of the 

 arm-vertebrae. Unfortunately his subdivision of these orders is not so 

 satisfactory, yet no one has attempted any essential improvement on it. 

 Whether the orders proposed by Bell represent natural groups which have a 

 definite place in the phylogenetic history of the brittle-stars has not yet 

 been critically determined; we can only say that they seem to be such. 



Although the chief reason why our classification of the brittle-stars is 

 so unsatisfactory is undoubtedly because no sustained attention has been 

 given to the problems involved, there are subsidiary reasons which should 

 not be overlooked. One of these is that the class shows a rather remarkable 

 morphological homogeneity so that the differential characters which un- 

 doubtedly exist are very considerably overshadowed by the extraordinary 

 development of species in the group, and as a consequence the systematic 

 work hitherto done has been almost exclusively concerned with the descrip- 

 tion of new forms. 



A second reason why so little progress has been made towards the 

 natural classification of the group is because the development of so few 

 species is known and in nearly all the known cases attention has been 

 centered on the larval structures or on the metamorphosis. Until very 

 recently the wealth of important data to be found in post-larval stages has 

 been almost wholly overlooked, largely, no doubt, because suitable material 

 is not easily obtained. But Ludwig (1899) and, more recently, Mortensen 

 (191 2) have made notable contributions to our knowledge of these most 

 interesting stages. 



A third reason why our progress has been so slow is the comparatively 

 small amount of paleontological evidence available, together with the 



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