MONOGRAPH OF THE EXISTING CHINOIDS. 11 



that the differences, trifling though they may seem, are really fundamental and 

 valid. 



Students of bilaterally symmetrical animals, especially those animals endowed 

 with powers of locomotion, are accustomed to a relatively large coefficient of specific 

 differentiation; this is true even among other groups of echinoderms in which the 

 individuals lead a more or less bilaterally active life. Also among radially sym- 

 metrical animals which move actively about specific differentiation is usually more 

 marked than among those of sedentary habits. 



The difficulty of at first comprehending the comatulid characters is a difficulty 

 of comparative perception, not of fact, and is entirely due to a superficial similarity 

 in the gross anatomy and form. 



One can never tell without a most detailed inquiry what are good systematic 

 characters and what are not; the most obscure anatomical features often prove to 

 be of the greatest interest, while in the embryology even such points as the unequal 

 division of the ovum, as well as the absence in certain cases of the anterior tuft of 

 cilia, and the difference in size of the cells at the animal and vegetative poles of the 

 blastosphere, appear to be of specific significance. 



It is very important that systematists should consider all these points of 

 apparent difference, especially those which loom up large in the embryo but which 

 disappear more or less in the adults; it is also important that embryologists and 

 anatomists, aroused to a high pitch of enthusiasm over the discovery of certain 

 peculiarities in their material not previously noticed, should not be led either into 

 condemning the work of their predecessors as careless, or into arguing, from a wide 

 anatomical difference between two forms, a correspondingly wide systematic 

 difference. 



It is a common fault in works of monographic scope In magnify the systematic 

 side of the subject to the great detriment of the morphological; but a thorough 

 understanding of the anatomy and development of the animals of any group is 

 absolutely essential before the systematic aspect can be intelligently studied. 

 Diverse interpretations of different structures or organs by several authors have 

 often led to corresponding variations in their systematic treatment, variations 

 which have been difficult to appreciate hi their true proportions, because of neglect 

 to explain in advance the position taken. 



As a general rule systematists are inclined to attach altogether too little impor- 

 tance to anatomical or embryological features, and morphologists altogether too 

 much. For instance, P. H. Carpenter, as a systematist, passed lightly over the 

 peculiarities of the bracliial muscles in different forms, wlu'le as a morphologist he 

 greatly exaggerated the importance of interradials in the genus Tliaumatocrinus. 



I have been able to add but little to what has been done by previous workers in 

 the field of development and anatomy; but it is essential that these be explained 

 in some detail before the systematic treatment can be commenced. Instead of 

 giving an account of these phases of the subject taken from a comparative study of 

 the works of others, I have preferred to quote more or less directly from the leading 

 authors on the various points considered, giving full crcilit to them, and thus mak- 

 ing a far more satisfactory whole. Xo attempt is herein made to give an exhaustive 



