PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 245 
The closing of the siphon rim, however, is more than a mere 
puckering together of its edge due to the action of circular muscle 
fibers. The rim is not only pulled together, but is also drawn 
down toward the body of the siphon. This is due to the action 
of longitudinal muscle fibers which, in the siphon, lie nearer the 
cavity of the siphon than do the circular muscles. 
The siphon rim in Ascidia is so opaque that it was impossible 
actually to observe the action of the two sets of muscles. A 
transparent species Ecteinascidia turbinata, furnished the desired 
opportunity. Individuals two or three days old, measuring 
three to four millimeters in length, can readily be examined 
Fig. 6 End-on view of a partially closed oral siphon, showing the geometric 
arrangement of the folds in the test. The left side of the body is uppermost. 
with the low power of the microscope. These animals show the 
two factors of siphon closure beautifully. At first the circular 
(sphincter) muscles contract and partially close the rim. This 
is followed by a contraction of the longitudinal fibers, which 
results in a drawing in of the rim, thereby completing the closure. 
In these young individuals I have frequently observed the 
longitudinal muscles of the oral siphon contract so vigorously 
that the upper portion of the siphon was completely inverted and 
tucked into the branchial cavity. In Ascidia this retraction is 
provided for by a sudden decrease in the thickness of the test 
near the rim (fig. 10). The combined action of the two sets of 
muscles results in a closure which is really complete. No trace 
of a water current can be demonstrated after the siphons have 
been shut. 
