606 THE AMERICAN NATURALIST [Vol. XLII 



Whether we accept the annelids, or amphioxus, or still 

 other animals as representatives of the ancestors of the 

 vertebrates, we shall find it necessary, according to the 

 Balfour theory, to assume some intermediate form in 

 which the eyes are so located as to represent a condition 

 between a primitive integumentary eye and one such as 

 is possessed by the true vertebrates. Such a condition 

 occurs apparently only in the minute larvss of the 

 ascidians and in amphioxus. As very little is known 

 about the actual conditions in the ascidians in this re- 

 spect, we shall turn our attention to the optic organs of 

 amphioxus. 



Great diversity of opinion has been expressed as to the 

 parts of amphioxus that are sensitive to light. From 

 recent experimental work, however, it may be taken as 



fairly well established that the so-called eye-spot at the 

 anterior end of the nerve tube and the integument of 

 amphioxus are not sensitive to light, but that the small 

 pigment cups described by Hesse (1898) as photo-recep- 

 tive organs and found through almost the whole length 

 of the nerve tube are the true visual organs. They lie 

 in the substance of the nerve tube and though they have 

 not been regarded by Hesse as in any way connected 

 with the vertebrate eyes, they occupy a position that is 

 very suggestive of a stage intermediate between an ex- 

 ternal eye and one such as the vertebrate now possesses. 



