PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 263 
resulting closure depends on the intensity of the stimulus and on 
its distance from the siphon. After a short interval the siphon 
rim opens and the animal is normal again. 
2) If, however, the stimulus has been stronger, not only does 
the siphon rim nearer the stimulated area close, but the other 
siphon rim also closes. A new set of muscles has been called into 
play. 
This reaction is to be differentiated from the one in which both 
siphons are stimulated, such as when a drop of water is allowed to 
fall on the surface of the water in the aquarium. In this case 
each siphon is independently stimulated by the same disturbance. 
Such a reaction persists when all nervous connections between 
the two siphons have been cut (cf. Loeb, ’92 and Magnus, ’02). 
It is otherwise with the response which I have described. Nor- 
mally, if one siphon is touched so carefully that the animal is 
not jarred, both siphons will close provided the proper intensity 
of stimulus is used. When, however, the nervous connections 
between the two siphons is severed, only the stimulated siphon 
rim contracts. 
3) In response to an ordinarily vigorous mechanical stimulus, 
A. atra reacts by the employment of still an additional set of 
effectors, the longitudinal muscles of the body. Not only do 
both siphons close, but the body bends on its long axis toward 
the right side. 
This bending toward a structurely determined side is of signifi- 
cance in the ecology of Ascidia. Most individuals of the species 
are attached with the body projecting at any angle, but mainly 
in a nearly horizontal plane. All such animals which I examined, 
were found with the left side of the body uppermost. Conse- 
quently, the curving toward the right side results in bringing the 
siphons into such a position, that a disturbing body on the outside 
will roll off, and one on the inside will fall out. 
In the previous work on ascidians the reaction which involves 
the bending of the body has been the only one which has received 
any adequate attention. It has been generally regarded as the 
only reflex of which this group of animals is capable, and there- 
