STUDIES ON AMPH1OXUS 

 BY E. L. MARK AND W. J. CROZIER 



51. I. The photoreceptors of amphioxus * W. J. CROZIER, Bermuda Bio- 

 logical Station. 



An incandescent filament appropriately mounted was substituted 

 for the ocular on one limb of a binocular microscope; by means of this 

 device it was possible to secure, with properly adjusted diaphragms, 

 an intense beam of light, microscopic in diameter, which was focussed 

 within or upon various portions of Branchiostoma carribaeum. The 

 exact location of the light-spot, and the extent of light scattering by 

 the tissues, were observed through the other tube of the binocular. 

 Practically every portion of the body of a number of lancelets was 

 examined in this way in a dark-room; precautions were taken to avoid 

 mechanical stimulation, to which amphioxus is very sensitive in the 

 dark. No responses were obtained except when the light was focussed 

 upon or within the ventral half of the nerve cord. It was possible to 

 prove, notably by experiments with individuals in which portions of 

 the integument were thoroughly anaesthetised, that this stimulation 

 did not concern photoreceptors in the skin. 



The integument of amphioxus therefore contains no normal photo- 

 receptors. As indicated by Parker's less precise tests, the optic cups 

 within the nerve-tube are probably the light-sensitive organs in this 

 animal. This conclusion is substantiated by the details of the il- 

 lumination trials, and particularly by the demonstration of photo- 

 mechanical changes in the pigment cups of the "Sehzellen." Some 

 evidence was secured which points to the photosensitivity of the "dor- 

 sal Sehzellen" of Joseph. The region of the anterior pigment spot 

 is insensitive to light. 



2 Contribution from the Bermuda Biological Station for Research, No. 53. 



