270 SELIG HECHT 
small particle of sand is dropped carefully upon the tentacles, 
the slight back pressure produced by the closing of the atrial 
rim at once squirts the particle out of the oral siphon. 
In view of the certainty and ease with which these reactions 
may be demonstrated, not only in A. atra, but also in Hcteinas- 
cidia turbinata and in another unidentified species, it is dificult 
to understand why some authors have reported that the tenta- 
cles are practically insensitive to mechanical stimulation. Thus 
Roule (’84, p. 37), who studied Phallusia, and Lacaze-Duthiers 
et Delage (’99), who observed Cynthia, state that no noteworthy 
reaction occurs when the tentacles are touched in this way. 
This is all the more strange because it is precisely here that See- 
liger (189311, p. 323) has found most of the bristle cells to which 
he rather doubtfully ascribed the réle of touch receptors. 
The perception of mechanical irritation by the internal surface 
of the atrial siphon is of significance in the daily routine of the 
species. A decidedly sensitive area is at the bottom of the atrial 
cavity near the anus. The feces are discharged into this cavity. 
Here they furnish the mechanical stimulus for a reflex of the 
crossed type: the oral siphon closes and the body contracts, 
squirting the water and the feces out through the atrial siphon. 
To one unacquainted with the presence of the group of crossed 
reflexes, the defecation of Ascidia seems almost a conscious pro- 
cedure. It ‘tries’ to force out the feces, and if a piece becomes 
caught in the siphon rim or in the atrial cavity, it ‘tries’ again 
to dislodge it by means of the ejection reflex, until finally it 
succeeds. The whole process can, however, be called forth by 
placing a glass bead ora pebble in the atrial cavity, or by repeat- 
edly stimulating it with a glass rod. 
2. Vibration 
The extreme sensitivity of Ascidia to mechanical stimulation 
is manifested in its ability to respond to vibrations (compare 
Marage, 05). Ascidia lives in shallow water, and if the rocks 
within two or three meters of an individual are stamped upon 
with even a modicum of vigor, it closes its siphons. <A rapid 
motion in the water within the same range also serves as a 
