222 NEW YORK STATE MUSEUM 



Before leaving- the discussion of function we must note that it 

 raises a question as to the nature of the canal coverings. With 

 contractile b.vs., such as are here postulated, there must be 

 some provision for filling with fluid the space between the canal 

 wall and the b.v. during the contraction of the latter. \\'ith a 

 canal covering formed by an impervious sheet of epistereom, 

 the compensating fluid would have to be drawn from beneath the 

 theca and, as there is no evidence for any pores through the mes- 

 ostereom itself, such fluid would have to ascend in the vertical 

 canals and outside of the tubular walls of the b.vs. The larger 

 diameter of the sutural canals would allow them to serve such 

 a purpose. We have evidence, however, that the canal cover- 

 ings were not impervious and that water could enter the canal 

 from the outside on contraction of the b.v. within it. An exam- 

 ination of the ascending canals on the basal represented in plate 

 5, figures i and 2, shows a line of very porous epistereom lying 

 directly over the canal. \\'here the covering has been broken 

 away, as at a, this seam of porous epistereom *s seen to penetrate 

 to the canal itself. The thickness of this epistereom, and yet the 

 maintenance of the porous seam, is very decided evidence for the 

 respiratory nature of the structure. Text figures 4 and 5 show 

 this same feature and also suggest that with the external thick- 

 ening of the ridge the deeper layers of the porous epistereom 

 were absorbed, thus leaving a narrow slitlike cavity over the 

 b.vs. The thickening of the ridges was by growth on the outside 

 and a new sievelike epistereom was formed over the more porous 

 older material. Water evidently filtered through these lines 

 as through a madreporite. The study so far given seems 

 abundantly to justify the conclusion that wq are dealing with 

 respiratory processes and this conclusion is but strengthened 

 when we note that they form an elaborate system and cover all 

 the plates of the theca. 



Whether or not one accepts the interpretation here given, we 

 may note that the evidence so far presented is very decidedly 

 against certain former interpretations. These ridges are most 

 certainly not " axial folds of the plates " as others have called 

 them. They are not in any way indicative of nerve branching 

 to supply distant organs, nor do they indicate either " incipient 

 hydrospires " or the former possession of such structures. The 

 suggestion that we here have grooves for stroma strands or for 

 muscular processes is negatived by the deepening of the surface 



