448 PROCEEDINGS OF THE AMERICAN ACADEMY. 



such views of the origin of the lateral eyes of vertebrates as have been 

 advanced by Sharp ('85), Burckhardt (:02), and others, according to 

 which the lens is regarded as having been derived from the primitive 

 retina, now replaced by a photoreceptive differentiation of a deeper 

 ganglionic part. I agree with Boveri in looking upon the eye-cups 

 of amphioxus and, I may also add, the corresponding elements of the 

 tunicate eye as the forerunners of the vertebrate retina, and, though 

 I was at first inclined to ascribe to these a direct origin from the 

 external skin, I now believe that we at least have no good reason for 

 this assumption. 



The chemical sense is the only one in amphioxus that seems to 

 possess a well-marked special organ, the so-called olfactory pit, and 

 yet for this organ both Nagel's experiments and mine gave no signs 

 of sensitiveness other than that which characterizes the skin of the 

 anterior end. Notwithstanding this negative evidence, the morpho- 

 logical relations of this pit are such that I believe it is very probably 

 the homologue of the olfactory organ of the higher vertebrates. That 

 a special function has not been discovered for the olfactory pit in 

 amphioxus is perhaps not surprising when it is remembered that no 

 direct physiological evidence whatsoever is at hand bearing on the 

 function of the olfactory organs of fishes. That these organs are un- 

 doubtedly of great significance in the life of a fish is attested by the 

 extent of their surfaces and by the size of the connected parts of 

 the brain, and yet, so far as the habits of fishes are concerned, we 

 have no conclusive evidence as to their real uses. 



The outer surface of amphioxus is sensitive to a variety of sub- 

 stances, such as nitric acid, picric acid, alcohol, etc., and to all these 

 substances the animal responds by withdrawing. Nothing could be 

 discovered about its reactions that could lead to the belief that the 

 chemical sense was connected with feeding. This sensitiveness was 

 found in amphioxus to be dependent, not upon nerves from the region 

 of the mouth that had invaded the outer skin, as Herrick (:03) has 

 shown for many fishes, but upon the segmental nerves of the region 

 stimulated, for the posterior third of an amphioxus will react, like the 

 whole animal, to effective chemical stimuli. The chemical sense of 

 amphioxus is, then, not especially associated with its mouth or its 

 feeding habits, but is a general integumentary sense, the function of 

 which seems to be to help the animal to escape an unfavorable chemical 

 environment. Apparently this is the primitive function of the chem- 

 ical sense as it is met with in the skins of many animals, and this 

 unspecialized sense has afforded a basis from which in the region of 

 the mouth the specialized senses of smell and taste (both of which are 



