Preliminary Notes on Growth-Stages in 

 Brittle-Stars 



Arthur S. Campbell 



There are a number of conditions to account for our present 

 lack of a rational system of the brittle-stars. One of the principal 

 reasons why the group is so difficult to classify lies in the profound 

 ignorance of their growth-changes. The excellent systematic 

 work of Ljungman, Lutkin, Lyman, Koehler and the two Clarks 

 have brought some thousand species to attention but the real rela- 

 tionship of these as larger groups is yet quite unsolved. There have 

 been several attempts to rationalize the classification, one by Bell, 

 1892, and more recently by Matsumoto, 1915. Neither of these sys- 

 tems is thoroughly based upon phylogenetic history, and hence, can- 

 not be conclusive since the state of our present knowledge is such 

 as to forbid any sweeping generalizations. 



Although the chief reason for our lack of a rational system in 

 the group is this lack of attention upon growth-stages, another 

 lies in the general disregard of palaeontological evidence, and a fur- 

 ther reason because attention has been focused upon larval, rather 

 than post-larval, stages. 



Material heretofore studied in connection with this problem 

 of gi'owth-stages in the young of ophiurans numbers less than one 

 dozen species, all of which are Atlantic or West Indian forms. My 

 own observations were made upon seven species, the members of 

 five families. All are the members of the littoral fauna of Southern 

 California. Specimens were collected in all accessible habitats and 

 studied after preservation. 



The excellent plates for this paper are the work of Miss E. 

 Keyes, a student in Pomona College. 



It is not always possible to tell just why one places a form in 

 this or that group for many characters are subtile and one is obliged 

 to depend very often upon general features. Especially is one 

 dependent upon as complete a series as possible in placing a juve- 

 nile. H. L. Clark, in his paper on growth-changes in some brittle- 

 stars expresses the only formulation of the very important con- 

 tribution of R. T. Jackson to the study of juvenile brittle-stars that 

 I have seen. This law is a very real help in determining possible 

 relationships between specimens otherwise obscure or impossible 

 to differentiate. Briefly stated, we may say that, as applied to 

 these forms, the base of an arm of a young form corresponds ex- 

 ceedingly suggestively with the tip of an arm of an adult specimen 

 of the same species. However, the extent of localization varies 

 greatly in different species, as I have found. One needs much 

 .study to determine accurately the position of a given specimen. 



