NO. 1846. ON CERTAIN ELEUTHEROZOIG PELMATOZOA—KIRE. 119 



such ability was possessed by many of the Cmioidea, as we nia}' well 

 be safe in assuming, there are a number of reasons for such migration. 

 The most obvious reasons, perhaps, are those relating to the better- 

 ment of physical conditions and food supply. These two factors 

 are more or less mutually dependent, naturally. In a sea lacldng 

 currents, and at approximately the same depth, with uniform condi- 

 tions of sedimentation there would be little choice in location. Such 

 uniform conditions seldom if ever obtain, however, except in com- 

 paratively restricted areas. The contour of the sea bottom, par- 

 ticularly in the shallow epicontinental seas, must have varied some- 

 what, making certain areas more suitable for the crinoids than 

 others. Far more important, however, are the effects of currents, 

 the presence of which in the Paleozoic seas may be assumed with a 

 considerable degree of assurance. These currents would largely 

 affect the food supply, and the Crinoidea if able to orient themselves 

 in relation to such currents, might obtain optimum food conditions. 



It is quite conceivable that the ciliated larvas may not have settled 

 at a locality entirely favorable to an adult crinoid. Indeed, the spot 

 most suitable for the needs of the very young crinoid might well be 

 far from desirable for the adult. Again, it is quite possible that 

 during even the short life of a crinoid, conditions might so change as 

 to make an originally favorable area less congenial. Moreover, in 

 order to bring about a wider distribution of the Crinoidea, post-larval 

 locomotion would be a factor of very great value. The importance 

 of a wide distribution m contributing to the welfare of a group is at 

 once apparent. 



Bearing of detachment on distribution. — The distribution of the 

 Crinoidea is necessarily effected by movement of the animals them- 

 selves. The change of location takes place either in the larval or 

 the post-pentacrinoid stages of development. It has been consid- 

 ered hitherto that larval dispersal is the chief factor in controllmg 

 distribution. As indicated above we have a great deal of evidence 

 going to prove that an eleutherozoic habit is largely maintamed by 

 the adult crinoids. It is here held that this post-larval freedom is of 

 considerable importance in supplementmg the activity of the mero- 

 planktonic larvse. The facts of crinoid distribution, and particularly 

 of their segregation, argues strongly for considerable post-larval 

 shiftmg among the Crinoidea. These topics, as elsewhere noted, 

 will be discussed at length in a future paper. 



Bearing of detachment on struggle for existence. — A prime reason for 

 the maintenance and further perfection of eleutherozoic conditions 

 not only among the Pelmatozoa, but among the Echinoderma in 

 general, is in order to escape the intense struggle for existence that 

 obtains in the crowded colonies of the bottom. The ciliated larvse 

 are probably of far less importance as effecting distribution among 



