NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 



17 



The disappearance of the interbrachials is quite in line with the 

 progressive development of the crinoid skeleton through the pro- 

 gressive elimination of the less essential elements. 



Frequency at different depths 



Frequency at different temperatures 



Average depth 



Average temperature 



750 fathoms 

 52.5 ° Fahr. 



747 fathoms 

 5 i.o° Fahr. 



II. COLUMN 



1. Entire column present. „ t . „, , 



^ Bathymetric Thermal 



range range 



Apiocrinidae 565-940 36.7-38.1 



Phrynocrinidae " 508-703 38.1-40.0 



Bourgueticrinidse 62-2690 29.1-70.75 



Holopodidse 5-120 71.0 



Plicatocrinidae 266-2575 31. 1-43.9 



2. Original column discarded in early life. 



Bathymetric Thermal 



range range 



Pentacrinidse . 0-2900 28.7-80.0 



Whatever may be said of crinoids as a whole, or of echinoderms 

 as a class, the column is an essential feature of the structure of the 

 Articulata, to which all of the recent crinoids except those of the 

 family Plicatocrinidse belong, and of the Inadunata, which includes 

 that family. 



The absence of the column, or the atrophy and rejection of the 

 larval stem, therefore, is clear evidence of specialization. 



