PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 297 
3. Ascidia is sensitive to tactile stimulation. The regions of 
greatest sensitivity are the siphon rims and the oral tentacles. 
4. Vibrations through solid and liquid media affect Ascidia, 
although transmission through the seawater is the normal method 
of stimulation. The receptors are located in the lobes of the 
siphon rims. 
5. The records of the amplitude of contraction to regularly 
repeated mechanical stimulation show that the cessation of 
response after a time is due mainly to a fatigue of the receptor 
mechanism. 
6. The ‘ocelli’ of Ascidia are not organs for photo-reception. 
The animals are sensitive to light of very high intensity only, 
and the sense organs are located within the siphon near the 
oral tentacles. 
7. Ascidia is thermosensitive. It reacts to temperatures above 
32°C. and below 20°C. 
8. Its test is insensitive to light, heat and chemicals. 
9. The animals react to large changes in osmotic pressure, 
and to the presence of the following classes of substances in solu- 
tion: salts, acids, bases, alkaloids and anesthetics. Solutions 
of sugars do not stimulate, but saccharin gives a decided reaction. 
10. The liminal concentrations and the relative effectiveness 
of all these stimulating substances are very similar to those which 
have been demonstrated for other physiological activities. It is, 
therefore, suggested that the primary factor which converts a 
group of cells of Ascidia into chemical sense organs is their con- 
nection with an effector system. 
