PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 449 



chiefly concerned with food discrimination) have been differentiated. 

 This unspecialized chemical sense has been retained in the skin of the 

 frog and other amphibians and in the irritable mucous surfaces of the 

 higher vertebrates, but its chief representatives in the higher forms are 

 its derivatives, the senses of taste and of smell. Of these, amphioxus 

 possibly possesses the sense of smell. 



Amphioxus may, therefore, be said to be an animal that possesses 

 in potentia at least the sense organs of the vertebrates. Its outer 

 surface is provided with tactile organs, but it does not possess the 

 derivatives of these, the lateral-line organs and the ear. Its outer 

 surface also contains undifferentiated chemical sense organs, but it 

 cannot be said to have a sense of taste, and the only evidence of a 

 sense of smell is morphological. Its outer surface, like that of the 

 higher vertebrates, contains temperature organs. Amphioxus also has 

 in the walls of its nerve-tube photoreceptors, which may well be the 

 forerunners of the rod- and cone-cells of the vertebrate retina. It is 

 thus an animal of fundamental importance for the understanding of 

 the vertebrate sense organs. 



'■&'• 



9. Summary. 



1. Amphioxus is only very slightly sensitive to light. 



2. It responds to a rapid increase of light, but not to a rapid 

 decrease. 



3. The only known photoreceptors in amphioxus are the eye-cups 

 in the wall of the nerve-tube. 



4. Amphioxus is photokinetic and negatively phototropic. 



5. Amphioxus is stimulated by water warmer than that in which it 

 lives (31° C.) and is killed in water at 40° C. or higher. 



6. It is also stimulated by water colder than 31° C. and is killed by 

 lengthy exposure to water of 4° C. or lower. 



7. It is thermokinetic and negatively thermotropic. 



8. The outer surface of amphioxus, especially the oral hood and the 

 tentacular cirri, is sensitive to mechanical stimuli. 



9. Amphioxus is also sensitive to sound vibrations. 



10. It is thigmotropic, and slightly rheotropic and geotropic. 



11. The outer surface of amphioxus is sensitive to solutions of 

 nitric acid, potassic hydrate, picric acid, alcohol, and to strong ether, 

 chloroform, turpentine, oil of bergamot, and oil of rosemary, but not 

 to solutions of sugar. It is also stimulated by diluted sea water and 

 by fresh water. 



12. Amphioxus is negatively chemotropic. 



VOL. XLIII. — 29 



