PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 247 
The smaller contraction in figure 8 represents a siphon rim 
movement; all the other records are almost identical with it. 
Curve 7 in figure 11 is another example, and represents in addi- 
tion an analysis of the movement. The record is divisible into 
four distinct phases. The first two are the phases of contraction; 
the second two those of recovery. 
The first phase is an almost straight line, and represents, 
therefore, a comparatively rapid contraction of the siphon rim. 
Although of short duration, lasting about a second, this por- 
tion of the movement accomplishes nearly three-fourths of the 
total contraction. The remaining closure is made at a much 
slower rate, as is shown by the amplitude and duration of the 
second phase. The condition of maximum contraction is reached 
in approximately four seconds. Recovery begins almost im- 
mediately, and in the beginning is comparatively rapid. The 
third phase of the record may be defined as that portion which 
lies between the position of maximum contraction and the point 
where the curvature of the line changes so as to be convex to 
the time axis. It lasts nearly twice as long as the second phase, 
and includes about one-half the recovery of the siphon rim. The 
last phase of the movement is the longest, occupying nearly 
three-fourths of a minute. At the end of it the siphon rim has 
assumed the diameter which it had at the beginning of the 
contraction. 
An analysis of the time relations of the phases of two separate 
records of the same siphon rim is given in the accompanying 
table (table 2). The two movements were made under the 
same conditions within a few minutes of each other, and were 
produced by the same intensity of mechanical stimulus. The 
similarity in the resulting records is very evident. 
TABLE 2 
Siphon rim closure. Exp. VI.23.1 
DURATION OF PHASES, SECONDS 
1 2 3 4 
I LSE 3.4 5.3 36.8 
II 1.5 3.4 9.2 40.5 
