REPORT OF THE DIRECTOR I9IO 201 



become surrounded by the stereom of plate extension. In such a 

 system the coelomic or other fluids of the body would be carried out- 

 side of the theca and effect osmotic interchange with the sea water 

 in that position. This form of respiration might well be designated 

 as exothecal. It would seem that the demands of the environment 

 for a more solid thecal wall should make this latter path the more 

 certain of adoption, for the endothecal system must of necessity 

 mean a larger and, other things equal, a weaker test. 



While assuming the attached condition and developing a thecal 

 armor the Pelmatozoa no doubt depended largely on ciliary action 

 for food supply and perhaps in part for respiratory circulation. The 

 manifest advantage of muscular contraction in bringing about such 

 a circulation and the presence of a well-developed muscular system 

 should, however, lead us to expect that a muscular respiratory 

 system would be developed. A more or less rigid thecal wall would 

 offer some very serious obstacles for such a system to overcome and 

 the conditions surrounding such an attempt should be briefly 

 examined. 



Take first the simplest possible type of muscular endothecal 

 respiration. We will suppose a single endothecal sac to exist, that 

 this is filled with sea water and that osmotic exchange has reached 

 equilibrium. To continue the respiratory process it becomes neces- 

 sary to expel this water. With a flexible theca, closed at all other 

 points, this could be accomplished by contracting the whole or any 

 part of the body wall and so reducing its volume by an amount 

 exactly equal to the volume of water to be expelled. With a rigid 

 theca, under similar conditions, the expulsion of this water would 

 be beyond the power of any conceivable muscular organization and 

 this from purely physical and well-understood reasons. With a 

 second opening to the body cavity, be it mouth, anus, genital pore 

 or madreporite, the ejection of the water contained by the sac would 

 be an easy matter if only an exactly equal volume of water were 

 allowed to enter the body by one of the other openings. That the 

 respiration of Echinoderms involves more than one set of organs is 

 well known. What we should note here is that we have a mechanical 

 cause acting in past time that is in itself competent to bring about 

 this very condition. 



To insure a better understanding of the problem and to see more 

 clearly some of the relations involved, let us put the matter in a 

 slightly different form. When thecal walls have become solid it is 

 no longer possible to contract any organ save under one of the fol- 



