MONOGRAPH OF THE EXISTING CKINOIDS. 15 



The general absence of correlation between the several sets of organs and struc- 

 tures which collectively make up the comatulid whole most emphatically negatives 

 the idea that the occurrence of the same structural type in widely different groups 

 is the result of parallelism, and indicates that among the comatuMds as a class 

 there is a given number of more or less distinct and independent types of each organ 

 and structure, any one of which may be combmed with any one of the types of the 

 other organs and structures. 



RELATIVE STATUS OP THE RECENT CRINOIDS. 



Since the crinoids are the most nearly strictly sessile of all the animals in the 

 sea, and since their relation to their food supply is always essentially the same 

 no matter what diversity there may be in the chemical and physical nature of their 

 surroundings, the facts furnished by a study of the geographical and bathymetrical 

 distribution of the recent crinoids are of the greatest value in the detei mination of 

 former land connections, just as the facts brought out by a study of the fossil repre- 

 sentatives of the recent genera and species are of the greatest importance in tracing 

 out the extent and time of existence of the ancient seas. 



The remarkable paleeontological record of the crinoids, and the abundance of 

 fossil forms closely related to existing genera and species, will allow of an accurate 

 estimate in regard to the geological time when these land connections were estab- 

 Hshed, and when they became disrupted. 



A comparative study of the recent faunas and those of past ages will show at 

 what epoch certain land areas and certain deep channels were formed, as a result of 

 which genera of subsequent origin were unable to spread into territory previously 

 colonized by older forms; while at the same time it will tlirow much hght on the 

 geological age of the components of the deep sea fauna, showing that it is a complex 

 formed of representatives of all the most virile types which have existed in all of the 

 past horizons. 



By a careful study of the chemical and jjhysical conditions under which the 

 recent forms Uve, a deternunation of their relation to temperature, salinity, Ught, 

 currents, etc., we shall be able to learn much which will be of the greatest value in 

 ascertaining the exact conditions under which many ancient strata were laid down. 



I have reserved the discussion of all these interesting points, as well as of the 

 distribution, oecology, geological history, and the relationships of the recent crinoids 

 to their fossil representatives (including the facts brought out by a comparative 

 study of recent and fossil species belonging to the same genera) — in other words, 

 the bearing of our Ivnowledge of recent crinoids upon the data gathered from a 

 study of palaeontology — until the end of this work, not only because the general 

 conclusions find their most logical place after the consideration of all the special 

 features and the complete presentation of all the data, but also for the reason that, 

 as the treatment herein adopted is such a radical departure from any ti-eatment 

 heretofore proposed, and the number of new species is so very large, no general 

 discussion would be of value until after the systematic framework upon which it 

 is of necessity based has been thoroughly elucidated and made easy of comprehension. 



