PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 253 
with definite sets of muscles. Therefore, besides serving as an 
excellent covering for the soft internal parts, the test in addi- 
tion functions as an exoskeleton, on which depends the proper 
execution of the movements of the animals (Fol. ’08). 
3. Spontaneous movements 
In the continued observation of Ascidia under all sorts of con- 
ditions, it became evident that complete movements of the body 
and siphons often occurred when no apparent external stimulus 
was present. The animal is extremely sensitive to mechanical 
stimulation, and at first I was inclined to attribute these aberrant 
contractions to very slight movements of my body or of other 
people in the laboratory. Such an explanation was, however, 
abandoned when the same movements occurred under con- 
ditions which precluded this source of stimulation. 
Somewhat similar spontaneous contractions have been de- 
scribed for the cirri and oral hood of Amphioxus as the result 
of the mechanical stimulation of the cirri by the accumulation 
of particles of sand (Parker, ’08, p. 431). This explanation does 
not hold in the case of Ascidia. When animals which had been 
carefully washed were placed in filtered seawater, they continued 
to perform spontaneous movements. Moreover, animals with 
the test entirely removed and with the greater part of the siphon 
cut away, and consequently deprived of their sensory apparatus, 
still exhibited frequent contractions. The factors for their produc- 
tion, therefore, rest within the organism itself. 
The solution of the difficulty came when a record was kept of 
the appearances of the spontaneous contractions under con- 
ditions which excluded external stimulation. Several animals 
were placed in individual battery jars containing about five 
liters of filtered seawater. The jars rested on a heavy table 
placed on the concrete floor of an isolated house built directly 
on the rock of Agar’s Island. The animals were observed con- 
tinuously for an hour, and the time of each spontaneous con- 
traction was noted. Figure 13 gives a graphic account of two 
such animals. It is very evident that there is a rhythmic 
occurrence of the spontaneous movements. 
