THE WATER CURRENT PRODUCED BY ASCIDIA 

 ATRA LESUEURi 



SELIG HECHT 



ONE FIGURE 



1. Ascidia atra is a common tunicate at Bermuda,- living 

 attached to rocks, in most cases well under low water. In speci- 

 mens that were taken to the laboratory and allowed to remain 

 undisturbed until both siphons had opened, a vigorous current 

 could be demonstrated entering the oral, and leaving the atrial 

 siphon. The outgoing current seems always to be stronger 

 than the incoming current, probably because of the somewhat 

 smaller size of the atrial opening, the volume of water passing 

 through the two apertures being, of course, the same (Wallen- 

 gren, '05, p. 12). This difference in the velocities of the two 

 currents and the angle formed by the axes of the two expanded, 

 diverging siphons, are concerned with keeping the outgoing 

 water from being drawn in again with the incoming current. 



I have always found this current to be present until the clos- 

 ing of one or both of the siphons by some stimulus caused it 

 to stop;- ' The oral siphon is the more irritable and is more likely 

 to be closed first, if the stimulus is not very strong. The siphons 

 therefore act like valves, since they control, but in no way help 

 to produce, the current. This is maintained by the cilia which 

 line the lateral margins of the stigmata in the wall of the bran- 

 chial sac (Orton '13). In Ascidia mentula — a species closely 

 related to A. atra — Herdman ('99, p. 17) has estimated that 

 adults of medium size have 192,000 stigmata. Since there is 



' Contributions from the Bermuda Biological Station for Research, No. 42. 



^ My best thanks are due to Dr. E. L. Mark, and to the trustees of the Hum- 

 boldt Fund, for making it possible for me to stay at the Bermuda Station during 

 the summer of 1915. 



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