252 THE EFFECT OF TEMPERATURE 



markable than in the case of Proteus, in that pigment 

 cells are entirely absent from the skin of the lower side 

 of the normal Flounder.* 



The observations of List f upon certain Lamelli- 

 branch Molluscs afford evidence as to the effects both of 

 decrease and increase of illumination. List noticed 

 that various species of Mytilus (gallo-provincialis and 

 minimus), which had been collected in caves, were dis- 

 tinctly less pigmented than usual. In fact, those ob- 

 tained from the extreme ends of the two grottoes 

 underneath the ruined Palace of Donn'Anna in the 

 Bay of Naples were all of them pale or colourless. In- 

 dividuals of the same species were also found in the 

 dark underground tanks of the Zoological Station, and 

 here again they were little, if at all, pigmented. Speci- 

 mens of M. minimus were even found in the pipe 

 through which the water is pumped from the sea into 

 the Aquarium, and these were characterised by an ab- 

 solute want of pigmentation. The converse observa- 

 tions were made upon LitJiodomus dactylus. These 

 molluscs, which are usually concealed in borings in the 

 sand of the sea bottom half a metre deep, are pigmented 

 only at the tip of the foot and the edge of the anal 

 siphon, these being the only parts at all exposed to 

 light. After keeping specimens for a year in a glass 

 vessel exposed to daylight, however, the whole surface 

 of the anal siphon became coloured an intense red 

 brown, whilst the imperfect branchial siphon, the border 

 of the mantle, the whole of the foot, and the other ex- 

 posed parts, were pigmented also. 



*IUd., p. 767. 



f Arch, f . Entwickelungsmechanik, Bd. viii. p. 618, 1899. 



