448 



H. BOSCHMA. 



the same conditions always gave the same results of the colora- 

 tion of the skin. 



As is shown by Fig. i the transversal stripes are very clearly 

 visible when the animal with uncovered head is on a white back- 

 ground. The parts of the skin between these stripes have a 

 light gray color which is brightest immediately behind the stripes 



FIG. 3. Ptychozoon homalocephalum, another specimen than that of Figs. I and 2, 

 on a black background. About f nat. size. 



and somewhat darker before them. A sharp contrast with this 

 figure is that of Fig. 2. Here the animal is photographed on a 

 white background, but with covered head. Now the color of 

 the skin is almost uniformly dark gray whilst the transversal 

 dark stripes are more indistinct. Almost the same color is 

 assumed by Ptychozoon when it is surrounded by black (Fig. 3). 

 Then also the stripes are only faint and the whole surface is 

 almost uniformly dark gray. 



