256 SELIG HECHT 
performance of a rhythmic contraction, Ascidia passes through 
a period during which a certain strength of stimulus fails to call 
forth a movement of the body. However, as this refractory 
period passes by and the time approaches for the culmination 
of the next rhythmic movement, this same sub-liminal stimulus 
will call forth not only the usual siphon contraction, but also the 
body contraction in advance of its scheduled time. 
The function of these rhythmic movements is by no means 
clear. Their accomplishment of a partial discharge of the water 
in the branchial sac would be intelligible if something else be- 
sides the water were also expelled. Ascidia, however, possesses 
an effective mechanism for avoiding just such a necessity. The 
tentacles in the oral siphon screen out all but the smallest particles 
which come in with the water current. Everything that passes 
beyond them is incorporated into the food cord in the branchial 
sac. The particles which are large enough to touch the tentacles, 
set off a reaction that drives the water and the particles out of 
the cavity of the siphon. Substances in suspension either get 
into the branchial sac and stay there, or they are forced out at 
once. Therefore, the function commonly attributed to rhythmic 
movements in other animals (Redfield, ’17) cannot apply in the 
case of Ascidia atra. 
To call the movements a respiratory rhythm would also fail 
to explain their existence. Ascidia has a highly efficient respira- 
tory mechanism which moves large quantities of seawater. The 
renewal of the contents of the branchial sac by the rhythmic 
discharges would be of no significance compared to the continuous 
stream of water produced by the cilia. The relative infrequency 
of the discharge would also argue against a respiratory rhythm. 
The spontaneous movements in Ascidia are not an isolated 
instance. I have observed them in a colonial species, Ectein- 
ascidia turbinata, the individuals of which are about two centi- 
meters long and quite light in weight. When the animals are 
attached, the effect of the body contraction is solely to discharge 
the water. If, however, an individual is removed from its attach- 
ment and placed in a large jar of seawater, it jerks itself along 
the bottom in a manner that vividly recalls the behavior of Salpa 
