1908." 



NATURAL SCIENCES OF PHILADELPHIA. 415 



2. The same relation holds true with regard to the arrangement of 

 the viscera, i.e., lobes of the liver, stomach and intestines. 



3. Cytologically, however, the tissues of the young snail are quite 

 different from the older ones. As an instance of this the liver of the 

 young snail contains large cells laden with yolk, all of which has been 

 completely absorbed in the dwarf. 



In a pond near Geneva Brot^ found that Lymtuea had a malformation 

 on the columella that seemed to be correlate i with the presence of 

 Hydra viridis. If a snail be long dwarfed and later be put ijnder favor- 

 able conditions, the shell is often strangely distorted. The pond near 

 Geneva may have nearly dried up and suddenly filled up'again. All the 

 snails in the pond would be under unfavorable conditions and dwarfed. 

 The pond filling up would offer ideal conditions and the snails would 

 grow. 



Dr. Pilsbry informed the writer that he has noted cases of this sort. 



A question of great interest is, will a dwarf put under favorable con- 

 ditions "grow up"? In Lymncea many experiments seem to show 

 that a dwarf does not cease to grow, but rather ceases to grow fast. 

 If, however, the snail is put under favorable conditions it starts at once 

 to grow faster and may ''grow up." However, they seem ''deUcate" 

 and it is with difficulty that they are raised. 



In this section the writer has attempted to outline some of the points 

 that have certain bearings on the experiments to come. 



IV, Methods. 

 In the brief review of the experiments of various authors that has 

 been given, certain controllable factors were shown that would affect 

 in certain ways the growth of animals. Every author on a priori 

 grounds has assumed that (1) food supply will influence growth. It 

 was found by Hogg ('54) and by Semper (74) that (2) the volume of 

 water affected growth. Semper showed that (3) temperature also 

 was a factor that could not be neglected, and that (4) the number of 

 individuals reacted in some manner on one another. Willem ('96) 

 proved that (5) aeration of the water affected the growth of Lymncea, 

 even as Yung ('79) had previously observed for tadpoles. De Varigny 

 considered that a large (6) area on which a snail could crawl was bene- 

 ficial to growth. The effect of (7) light was recognized by Higgen- 

 bottom ('50) and by Yung ('80). \'ernon ('95-'99) completed this 

 list by adding to it a factor, (8) the chemical composition of the water. 



^ Cited from the Camh. Nat. Hist., Mollusks, p. S8. 



