PHYSIOLOGY OF SYNAPTULA HYDRIFORMIS 359 



('87) seems to have been impressed by Quatrefages' observa- 

 tions and tried several experiments on Synapta inhaerens and 

 S. digitata. He was, however, unable to discover any reaction 

 to photic stimuli. Sudden strong increase of light or sudden 

 shading gave no results. He was therefore induced to agree 

 with Baur and Hamann that these pigment spots were not true 

 eye-spots. 



Ludwig und Barthels ('91), on the contrary, are emphatic 

 in their statement that the so-called eyes in Synapta vitatta 

 and S. vivipara (Synaptula hydriformis) are undoubtedly sense 

 organs. Probably their assertion is based on morphological 

 grounds, though they do not so state. Clark ('98), too, says 

 that "there can be little doubt that in Synaptula vivipara these 

 eyes are actually of service as light-detecting organs," though 

 he reports no experiments to prove his position, merely remark- 

 ing: "That this covering (the mesodermal layer surrounding 

 the nerves to the eye-spots) may be affected by light is probable, 

 for its color is due to the pigment it contains." Yet he also 

 states, "There is no reason to assume that the pigment in other 

 parts of the body is any different from that around the eyes." 



b. Observational. It is a fact that in several of the lower 

 animals, eye-spots, so-called from morphological considerations, 

 have been clearly shown not to function as eyes at all (Nagel, 

 '96, p. 34). In other cases, although eyes are present as true 

 photoreceptors, other parts of the covering of the body are also 

 sensitive to light (Parker, '03). Whether the eye-spots in 

 Synaptula hydriformis are truly special photoreceptors, I have 

 as yet been unable to ascertain. One thing, however, is cer- 

 tain, the whole surface of the body is sensitive to light. 



This Synaptula is decidedly negatively phototropic. When 

 placed facing a window in the daytime or before an electric 

 lamp at night, it at once turns from the light and moves to the 

 opposite side of the dish in a fairly straight line. The changes 

 of body position in this orientation are exactly those figured 

 by Crozier ('14 b, p. 12, fig. 1) for Holothuria captiva. Con- 

 sequently Mast's ('11, p. 211) sweeping statements — "The 

 Echinoderms are peculiar in that they can move with any side 



