459 



the rapidity with which the change is accomplished, make it certain 

 that the coordination is not accomplished by means of nerves. 



2. The apriori argument in favor of the stimulus to contraction 

 being transmitted along the contracting walls of the vessels is quite 

 strong. From what avo know about contraction in muscles it seems 

 hard to believe that the cells of one region could contract without 

 stimulating the neighboring ones. As a matter of fact a contraction 

 of the blood vessels between the ampullae has also been observed, thus 

 showing that the contractility is present throughout the whole. vascu- 

 lar system. This makes it possible for the stimulus to be conveyed 

 from one ampulla to the next, and if it originated in some of the larger 

 vessels near the zoöids and travelled peripherally we can see how it 

 might bring about a coordination. In testing this hypothesis, however, 

 I have been unable to find any evidence in its favor. Thus in observ- 

 ing the beginnings of the systole of a number of ampullae connected 

 with the zoöids by only one vessel a regular sequence could not be ob- 

 served. When the blood pressure was strong the ampullae usually began 

 to contract strictly together, and when they were connected with the 

 rest of the colony only by long narrow vessels, so that the pressure was 

 comparatively weak, there were usually irregularities in the sequence 

 of the contracting ampullae that could not be correlated with their 

 distances from the colony. The rapid readjustment of the coordination 

 in isolated groups of ampullae also tells against this explanation, 

 though not so strongly, as against the nerve hypothesis. The strongest 

 evidence against the conduction theory is the occasional occurrence of 

 adjacent ampullae that are freely connected with the rest of the vascu- 

 lar system but which do not contract together. This is but rarely 

 encountered and has been usually seen in parts of the vascular system 

 where the changes in blood pressure were comparatively small. In the 

 most conspicuous instance seen the two ampullae were almost touching 

 each other but they regularly contracted alternately. In this case, 

 hen, conduction cannot explain the phenomena; and this, together 

 with the other evidence, shows that, in general, the importance of the 

 conduction of the stimulus along the contracting vessels is small when 

 compared with other coordinating agencies. 



3. There remains then the variations in blood pressure as the 

 principal, if not the only cause bringing about the coordination of the 

 contractions; and when examined in detail it appears possible to ex- 

 plain the whole of the normal phenomena, as well as most of the va- 

 riations, entirely by this principle. 



In the first place we have seen that the blood pressure does in- 

 fluence the ampullar contractions somewhat though not enough to dis- 



