﻿REPORT OF THE DIRECTOR I9IO 203 



p. 213], or into tubelike extensions of stereom similar to those pre- 

 sented by Caryocrinus ornatus. This power of rapid 

 withdrawal requires a higher muscular and nervous development. 

 If the sac is not attached to the stereom surrounding the pores it 

 may be withdrawn as a whole but if so attached it must possess 

 internal muscles reaching to its distal end and be withdrawn by in- 

 vagination. Here we have opened still other lines of specialization in 

 which we shall find these processes functioning as sense organs, as 

 organs for food capture, and as organs for locomotion. Second, 

 while retaining their high nervous and muscular development, these 

 structures might be protected by clusters or fringes of immovable 

 or movable spines such as we see so highly specialized in Asteroids 

 and Echinoids. Third, these structures might early seek protection 

 by means of porous coverings of epistereom. Under this con- 

 dition the external sacs or tubes would of necessity come to lie close 

 to the thecal surface or in special depressions on the latter. Such 

 respiratory processes should be distinguished by the term epithecal. 

 Development in this direction led to considerable complexity of 

 structure but it never favored complexity or specialization of func- 

 tion. This third direction was chosen by many cystids and crinoids 

 and we shall find interesting examples in Palaeocrinus and Palaeo- 

 cystites. The tendency toward the development of a solid theoal 

 wall by a creature living in an incompressible medium has thus led 

 to the concomitant development of a system or systems of hydraulic 

 structures whose evolution may profitably be studied from a purely 

 mechanical point of view. 



Respiration in Blastoidocrinus. At the time of publication of 

 my description of Blastoidocrinus, in Bulletin 107 of the New York 

 State Museum (1907), I had not seen Billings's type. Through 

 the kindness of the late Dr J. F. Whiteaves I have since been enabled 

 to give it long and careful study and it has yielded evidence of very 

 great interest concerning this question of respiration. From 

 numerous photomicrographs made of features presented by this 

 type, nine have been chosen to illustrate the present article and will 

 be found reproduced in plates 1 to 4 inclusive. It will be seen that 

 we are dealing with the second form of endothecal respiration which 

 we have tabulated and already briefly discussed. 



In figure 2 of plate 1 of this paper we have a side view, x 10, 

 of a pseudambulacrum from which the wing plates and nearly all 

 traces of the brachioles have been removed by natural processes. 



