MONOGRAPH OP THE EXISTING CKINOIDS. 11 



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



valid. . 



Students of bilaterally symmetrical animals, especially those ammals endowed 

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

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

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

 metrical anunals which move actively about specific differentiation is usually more 

 marked than among those of sedentary habits. 



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

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

 in the gross anatomy and form. 



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

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

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

 division of the ovum, as well as the absence m certam cases of the anterior tuft of 

 ciha, and the difference in size of the cells at the ammal and vegetative poles of the 

 blastosphere, appear to be of specific significance. 



It is very important that systematists should consider all these pomts 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 

 pecuUarities in then- material not previously noticed, should not be led either into 

 condemning the work of theh predecessors as careless, or mto arguing, from a wide 

 anatomical difference between two forms, a correspondingly wide systematic 



difference. -, , 



It is a common fault in works of nionograpliic scope to magmfy 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 mtelUgently studied. 

 Diverse interpretations of different structures or organs by several authors have 

 often led to corresponcUng variations in their systematic treatment, variations 

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

 to explain in advance the position taken. 



As a general rule svstematists are inclined to attach altogether too httle impor- 

 tance to anatomical or embryological features, and morphologists altogether too 

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

 pecuharities of the bracliial muscles in different forms, while as a morphologist he 

 greatly exaggerated the importance of interradials in the genus Thaumatocnnus. 

 I haveljeen able to add but Httle to what has been done by previous workers m 

 the field of development and anatomy; but it is essential that these bo 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 othc<rs, I have preferred to quote more or less directly from the leading 

 authors on the various pomts considered, givmg full credit to them, and thus mak- 

 ing a far more satisfactory whole. No attempt is herein made to give an exhaustive 



