INTEGUMENTARY PHOTOSENSITIVITY IN A MARINE FISH 273 



the photoreceptors, resulting from the special conditions of translucency 

 in Amphioxus, may have had no permanent effect on the orientation of 

 these receptors in vertebrates generally. A completely satisfactory 

 determination of the phylogenetic significance of these cells seems to 

 involve the question of the existence and nature of similar sensory 

 cells within the neural tubes of other vertebrates. 



If, hypothetically, according to the Balfour theory, the integument in 

 phylogenetic development gradually loses its sensitivity to light, the 

 cord should by so much gain this property and, as the eyes develop, 

 the cord should in turn lose it, becoming specialized for the coordina- 

 tion and transmission of impulses rather than for receiving them. Of 

 course, these two evolutionary processes might not have been abso- 

 lutely continuous, and the conditions in the hamlet and some other 

 vertebrates indicate that they may not have been entirely dependent 

 on each other. It is, however, somewhat against Balfour's theory that 

 the degeneration of integumentary photosensitivity does not more per- 

 fectly parallel the evolution of directive eyes in the cord, 10 and also 

 that the characteristic vertebrate eyes are coexistent with a pronounced 

 cutaneous sensitivity in certain fishes, amphibia and reptiles. The 

 probable difference in the structure of the photoreceptive elements in 

 the skin and in the central nervous organs is also, as Parker ('05, p. 

 418) has pointed out, another possible objection. 



The idea of the in situ development of the retinal elements in the 

 neural tube and their non-derivation from primitively functional cutan- 

 eous elements received corroboration from Parker ('09) when he tested 

 several marine fishes and found the skins of all of them to be insensi- 

 tive to light. This he interpreted to mean that the vertebrate eye 

 developed not from integumentary elements, but, as Lankester ( '80) and 

 Boveri ('04) had suggested, in a manner similar to the in situ origin of 

 this organ in ascidian larvae. 



The present paper, in demonstrating the existence of cutaneous pho- 

 tosensitivity in one marine fish, makes this argument less convincing; 

 but the presence of sensitivity here, forming at present an exception 

 among marine fishes, affords only a suggestion of the importance of 

 further study on the origin of the eye of vertebrates and its relation 

 to the photosensitivity of the skin. The inconstancy of integumentary 

 photosensitivity may be taken as an argument for its secondary acqui- 

 sition and against its being a remnant of a primitive function. A final 



* 



10 Crozier ('17) has recently shown that the integument of Amphioxus con- 

 tains no normal photoreceptors. 



