40 Experime7ital Zoology 



Experiments with Flatfish 



There are some experiments made by Cunningham with 

 young flatfish which also show the effect of an external agent, 

 viz., Hght, on the development of color. Very young fish were 

 put into aquaria Hghted from below. As the young fish under- 

 went their metamorphosis, the pigment gradually disappeared 

 on one side, as it does under normal conditions, i.e. as when 

 they are Hghted from above only. If, however, the fish are 

 still illuminated from below, the pigment begins to come back 

 again on the lower surface. The markings are similar in all 

 respects to those on the upper side of the animal. 



The result shows that the lower side of flatfish in their natu- 

 ral environment is white, because it is not exposed to light ; but 

 whether the result shows, as Cunningham beheves, that the 

 lower side has become white in the course of generations, be- 

 cause it has been turned away from the hght, is not shown 

 conclusively by the experiment. 



Experiments with Crustaceans 



There are a few cases, in other groups, where it has been 

 shown that external agents produce changes in form. Bran- 

 chippus ferox, inhabiting salt and fresh water, shows small 

 differences in the length of the ovigerous sac, in the form of the 

 segments of the body, in the length of the lobes that terminate 

 the abdomen, and in the disposition of the abdominal bristles. 

 Daphnia degenerata of salt water is only a degenerate variety 

 of Daphnia magna of fresh water. 



A species related to Daphnia — Moina rectivoctris — oc- 

 curs in one form in fresh water and in another in brackish. 

 The two differ in general points of structure due to the indi- 

 viduals becoming sexually mature before the final structural 

 changes are completed. 



The oft- cited case of Artemia may also be mentioned here. 

 Schmankewitsch has described the slow transformation of 

 Artemia salina into A. milhausenii, as the lagoon in which the 



