337 



Id the facetted eyes of Arthropods it is generally believed that 

 the function of the pigment between the ommatidia is to absorb such 

 rays as do not enter in a direction parallel to the long axis of the 

 ommatidia. That the pigment has anything to do with the direct 

 stimulation of the end organs proper has not been shown. 



On the other hand, the presence of pigment associated with ordi- 

 nary cells, in such cases as the Medusa eye (or the stigmata of Proto- 

 zoa and Protophyta?), would indicate that the pigment may be con- 

 nected with the stimulation of the essential visual cells. 



Since pigment is not essential to render protoplasm sensitive to 

 light (heliotropism), and since we do not know of any definite relation 

 between the visual pigments and vision in the Vertebrate eye, are we 

 not justified in believing that such of the smaller eyes in the Salpidse 

 as have no pigment are, still, probably, functional optic organs? What, 

 then, is the function of the pigment which is found in connection with 

 the larger eye of Salpa, and, in one species, in connection with one 

 of the smaller eyes also? Göpfert^) suggests that, as is thought 

 to be the case in the facetted eyes of Arthropods, it serves as a dark 

 curtain to shut off rays from certain directions from reaching the rod 

 cells, thus enabling the animal to perceive direction. This seems to 

 me probable. Patten^) has suggested that the pigment in many 

 eyes may serve to absorb energy from the light, and that it may, in 

 certain cases, have no essential connection with vision. I do not 

 know of any experimental evidence in favor of this view. I do believe 

 that the morphology of the smaller eyes of Salpa indicate that pig- 

 ment is not essential to vision. May it not be possible that in cases 

 where we have pigment spots associated with ordinary nerve cells 

 (Medusa eye), or with ordinary ectoderm cells (earthworm prostomium 

 and mantle-edge of Lamellibranchs), the light absorbed by the pigment, 

 being converted through chemical action into heat, may warm the 

 cells near by, thus rendering them more sensitive to any kind of 

 stimulus, light waves included? I do not wish to emphasize but 

 merely to suggest the possibility of such a function for the pigment. 

 This use of the pigment may coexist with its function as a dark 

 curtain, where the pigment layer is properly arranged ^). 



1) E. GöppEET, Untersuchungen über das Sehorgan der Salpen. Morph. 

 Jahrbuch, Bd. 19, Heft 3. 



2) Wm. Patten, Eyes of Molluscs and Arthropods. Mitteilungen aus 

 der Zoologiechen Station zu Neapel, Bd. 6, 1886. 



3) It would be a simple matter for anyone, who can obtain live 

 individuals of the chain form of S. pinnata, S. cylindrica (?), S. hexagona, 



