PHYSIOLOGY OF ASCIDIA ATRA LESUEUR PAS 
obtained from the same individual, curve 6 is the result of a 
stimulus twice as intense as those whose records are given in the 
table, and consequently, the two phases last considerably longer. 
Whereas the actual bending of the body involves merely the 
action of the longitudinal muscles, the recovery to the normal 
shape depends upon several factors. One of these is the activity 
of a set of muscles situated on the left side of the siphons.  Al- 
though these muscles are not well developed, and extend for a 
short distance only, they act probably like extensor muscles, 
and tend to antagonize the action of the long muscle strands of 
the right side. A second factor is the elasticity of the soft tissue 
of the body. That these two agencies alone are capable of 
bringing an animal back to normal shape is shown by the com- 
plete, though slower, recovery of animals with the test entirely 
removed. 
The test, however, is a structure of considerable significance 
in the resumption of the normal form after the bending of the 
animal. Although apparently homogeneous, the cellulose ma- 
terial of ascidians has been shown to possess a fibrillar structure 
visible in polarized light (Schulze, ’63). Probably this hetero- 
geneity, which is also to be seen in some stained preparations of 
A. atra, is of importance for the elastic properties of the test. 
Its peculiarities resemble those of a viscous solid. To a sudden 
distortion, the test will respond in a manner comparable to most 
elastic bodies. If, however, it is subjected to a slow distortion, 
it will partially accommodate itself to the new form, and never 
return to the original shape. 
In the bending of Ascidia the activity is sufficiently rapid to 
cause an immediate elastic recoil on the part of the test. Sec- 
tions of the body show that the resilience of the test is utilized 
to good advantage (figs. 10 and 12). The left side is noticeably 
thicker than the right and, consequently, serves as an elastic 
back which antagonizes the muscles of the right side. Although 
controlled by the relaxation of these muscles, the elastic rebound 
of the left side probably serves in a large measure to straighten 
the curved body. 
