238 SELIG HECHT 
therefrom the substances necessary for the existence of the spe- 
cies. Both of these processes are accomplished in the branchial 
sac. The enormous development of this structure testifies to 
the necessity of working on a large scale in order to abstract 
the relatively meager proportion of food and energy contained in 
the seawater. 
1. Water current 
A study of the water current (Hecht, 716) has already shown 
that this form of activity has the following properties. The 
current is produced by the cilia of the branchial sac. It is 
maintained under a low pressure of 1.7 mm. of seawater. The 
quantities of water moved are large; in a medium sized individual, 
173 liters of seawater are transported in a day. The volume of 
water moved per unit body weight varies inversely as the size 
of the animal. 
Since the water enters by way of the oral siphon, and leaves 
through the atrial siphon, it is of primary importance to Ascidia 
to avoid a mixing of the incoming and outgoing currents. In the 
open water the movements of the sea undoubtedly change the 
water immediately surrounding an individual, so that a fresh 
supply of seawater is frequently available. Ascidia, however, 
does not rely on such a chance renewal of its food and energy 
. supply, because even in very quiet water, such as that in a large 
dish in the laboratory, the two water currents are definitely iso- 
lated from each other. 
If, in such a dish, particles of carmine are floated near the 
atrial and oral siphons, it is at once apparent that the outgoing 
current is considerably stronger than the incoming current. 
Figure 3 shows a drawing of a small specimen of Ascidia life size. 
The arrows near the oral siphon indicate the range of its activity, 
that is, the distance from the opening within which a particle 
of carmine was sucked into the cavity. For this specimen the 
distance was at most 5 millimeters. The arrow pointing away 
from the atrial siphon represents the distance within which a 
particle was deflected by the outgoing current. This range was 
ca. 65 millimeters. Other individuals showed similar relations 
