202 NEW YORK STATE MUSEUM 



lowing conditions. Contraction of one organ is possible if its incom- 

 pressible contents can be made to pass more or less completely to 

 another organ or body cavity. If the contents of an organ are to be 

 passed outside of the theca it can only be accomplished by admitting 

 an equal volume of sea water to some other organ. To illustrate 

 this reciprocal action, let us suppose that it is desired to draw water 

 through the madreporite. This may be accomplished by reducing 

 the amount in the alimentary canal. If on the other hand it is 

 desired to reduce rapidly the amount of water in the hydro-vascular 

 system, this may be accomplished by its contraction and the passive 

 admission of water to the alimentary canal. The largest external 

 opening possessed by Pelmatozoa is the anus, and this, if not closed, 

 would become a compensating tide-w^ay allowing contraction of any 

 other body cavity. Such a function at once makes the anus more or 

 less of a respiratory center and in some Echinoderms it has become 

 highly specialized as such. 



Passing from the case of one special subtegmental respiratory sac 

 to one of two or more we will readily see that the ejection of the 

 contents of one would mean simply the filling of another if only it 

 were passive at the time, and that a long series of such structures 

 could be emptied and filled by a rhythmical and progressive or 

 peristaltic contraction. It must be borne in mind, however, that no 

 matter how complex such a system may become the contraction of 

 any portion of it will be felt immediately by all other organs and 

 the tendency to make these others auxiliary organs of respiration 

 will be always present. This tendency is of course controlled by 

 natural selection and the adaptations secured are varied and often 

 present a very high degree of specialization. 



The other path, or that of exothecal respiration, presents no 

 exception to the principles already stated. The contraction of any 

 one of these exothecal sacs would be impossible unless its contained 

 coelomic or other fluid was allowed to flow back under the theca 

 and such a flow would be impossible unless fluids already in that 

 position were allowed to distend other exothecal sacs or were dis- 

 charged directly into the sea and thus lost to the organism. 



Specialization of exothecal respiratory processes. Such exo- 

 thecal sacs as we have just discussed, whether papullae or podia, 

 involve a new series of adjustments to environment, for their posi- 

 tion renders them liable to attack from other creatures. Protection 

 may be secured in three directions. First, by the power of rapid 

 withdrawal into or under the plate, as in Cleiocrinus [see fig. 2 



