No. I.] THE METAMERISM OF NEPHELIS. 2$ 



The color of the adults varies from a light chocolate brown 

 free from any mark of pigmentation to almost a coal black free 

 from any light areas. Between these extremes of very light 

 and very dark all gradations of color and varieties of pigmenta- 

 tion may be found in individuals collected in the same pond or 

 stream. The very light adults are comparatively rare, while 

 among the young smaller individuals unpigmented specimens 

 are quite common. The very dark adults are about as 

 frequent as the very light adults, while a young dark individual 

 is very rare. Most of the individuals that I have collected 

 would fall into two sorts : those in which the pigmentation is 

 diffuse, varying only in intensity through many shades, and 

 those in which the pigmentation is arranged to a greater or 

 less degree in longitudinal stripes. I have collected three 

 individuals that showed definite pigmentation on the first ring 

 of each somite, such as Blanchard (4) describes as constant for 

 JV. octoculata. Two were from Coonamasset pond near Woods 

 Holl, and one was from Wolf Lake near Chicago. Other speci- 

 mens with the diffuse type of pigmentation have shown a 

 slightly accented color on the first rings of some of the 

 somites, but not to the extent of defining all the somites. 

 The ground color is either a light mahogany brown or a pale 

 plumbeous gray. This may be observed on the ventral side, 

 which is usually free from pigment. The color of an indi- 

 vidual depends upon the amount of dark opaque pigment 

 present either as small granular particles or as highly branch- 

 ing pigment cells. If the view of Graf (8) is correct, that the 

 chloragogen cells wander into the epidermis and there break 

 up, leaving their remains as pigment particles, then the wide 

 variation in individuals taken from the same locality may be 

 explained as individual variations in the manner of excretion. 

 It is interesting to note that the stripes of color so common 

 in Nephelis lie in the lines of least resistance for wandering 

 chloragogen cells. A reference to PI. VIII, Fig. 18, shows five 

 spaces between the bundles of long muscles on the dorsal side 

 through which pass the dorso-ventral muscles, nerves, and 



8 Arnold Graf : " Beitrage zur Kenntniss der Exkretionsorgane von Nephelis 

 vulgaris." Jenaische Zeitschrift fiir Wissenschaft, N. F., Bd. xxi, p. 163, 1893. 



