458 



Coordination of til e Pulsations. 



The coordination of the ampullar contractions is their most 

 important characteristic, for it is only by means of this working to- 

 gether that they gain enough strength to force the blood rapidly through 

 the larger vessels and thus, of their own accord, maintain a vigorous 

 circulation. This coordination is often very extensive, all the ampullae 

 (perhaps 50 or more) in 4 or 5 square millimeters pulsating together; 

 but it is not usually more extensive than this. 



In attempting to ascertain the means by which this coordination 

 is accomplished there are three principal possibilities to be tested. The 

 coordination may be accomplished by: 



1. Nervous control. 



2. Conduction of the stimulus along the vessels, each contracting 

 cell stimulating its neighbor. 



3. A similar blood pressure in the coordinated region, the pressure 

 having an inhibiting or stimulating action or both, according 

 to the circumstances. 



1. The meagre development of the nerves in Botryllus^ and the 

 fact that they have never been observed to extend far from the ganglion 

 or into the common test makes it improbable that these should be the 

 cause of the coordination, and the behavior of the ampullae when 

 severed from the rest of the colony conclusively proves it. Thus when 

 a mass of ampullae is severed from the colony their contractions be- 

 come coordinated in a few hours, at most, but not in the same way 

 that they were before. They were all acting together then, driving the 

 blood towards the zoöids or drawing it away, while now they have divi- 

 ded into groups which contract alternately so that the blood is kept 

 circulating between them (Fig. 2). When first separated from the co- 

 lony many of the ampullae are separated from each other. They send 

 out processes which fuse and form small groups of ampullae whose con- 

 tractions are alternately coordinated. Later these groups fuse to form 

 larger ones and again we have another coordination supplanting the 

 old one, and better adapted to the purpose of keeping up the most 

 vigorous circulation. The rapidity with which this readjustment 

 may take place is surprising. Thus in isolating several pairs of 

 ampullae, which previously contracted together, the first alternate con- 

 tractions were observed 7 minutes after isolation, and in another case 

 well coordinated alternate contractions following each other at the 

 usual intervals were established 18 minutes after isolation. These facts, 

 especially the change of coordination when isolated groups fuse, and 



