WATER CURRENT PRODUCED BY ASCIDIA ATRA 433 



III 



Of equal interest with the volumes of water found for the 

 currents in Ascidia, is the pressure at which these currents are 

 maintained. This I determined in the following manner. 



A glass tube was tied into the atrial siphon and the animal 

 placed so that the tube was vertical Enough sea-water was 

 added to cover the animal and the free end of the tube. After 

 a few minutes the oral siphon opened, and a disturbance of the 

 surface of the water near the tube showed the presence of a 

 current through the branchial sac. The level of the water in 

 the jar was then lowered very slowly by means of a siphon, until 

 the height in the tube was about a centimeter below the pro- 

 jecting end. The height of the water in the tube above that of 

 the water in the jar was then measured. The level in the jar 

 was again lowered, and again the height in the tube measured. 

 This was done five times for each animal. Care was taken 

 to see that the oral siphon remained open and submerged during 

 the operation. 



After the fifth determination, the animal was removed from 

 its attachment to the tube, the latter retaining its position. The 

 jar was filled with sea-water and similarly five measurements 

 were made of the height of the water in the unattached tube. 

 The difference between the two sets of readings gives the cur- 

 rent pressure in millimeters of sea-water. 



The pressures thus found are slightly lower than those ob- 

 tained by Parker for most of the sponges. An average of ten 

 determinations on Ascidia (specimens of ca. 100 gms. each) 

 gives the pressures as 1.7 mm. of sea-water, the range of variation 

 being from 1 mm. to 2.1 mm. 



IV 



These observations show that the quantity of energy devoted 

 in Ascidia atra toward the maintenance of a sea-water current 

 is distributed, in the light of the functions of such a current, in 

 an efficient manner. A current pressure very much in excess 

 of that necessary to overcome the inertia of the water, would 



