188 University of California Publications. [zoology. 



Specific gravity determinations were made on fifteen individ- 

 uals. Gum arabic and white of egg were used to increase the 

 specific gravity of sea water to correspond to that of the organism, 

 according to the method employed by Miss Piatt. '97 p. 121 foot- 

 note, and '99 pp. 31-28. Gum arabic was found to be somewhat 

 more satisfactory than white of egg owing to its greater specific 

 gravity, although for animals having small specific gravity the 

 results were identical for both media. 



For convenience of comparison the results may be divided 

 into three groups. Group I will contain young tornaria of the 

 latter part of the first period; group II, those having reached 

 maximum larval development, from 2.00-2.60 mm. in length, 

 corresponding to the secoud period; and group III, those some- 

 what advanced into the third or metamorphosing period. The 

 averages are tabulated as follows: 



Group I Group II Group III 



(5 individuals) (7 individuals) (3 individuals) 



Specific gravity 1.038 1.033 1.069 



The "overweight," Ostwald, '02, representing the force by 

 which a plankton organism is di*awn downward and which must 

 be overcome if the organism is to rise and maintain itself in the 

 water, is expressed by the difference between the specific gravity 

 of the organism and that of the medium in which it floats. The 

 differences in overweight are relatively somewhat greater than 

 indicated by the above specific gravities, owing to a slight vari- 

 ation of the specific gravity of the water in which the individuals 

 of the three groups are living. The overweights are as follows: 

 Group I, .01-47; Group II, .0097; Group III, .<>4.~>7. 



Late metamorphosing stages sank rapidly to the bottom of 

 the aquarium. The specific gravity of these was too great 

 to be determined by the method used for the other stages. The 

 relation of specific gravity to age of tornaria is approximately 

 represented in text figure 1, on the next page. 



That active movements are mainly concerned in overcoming 

 the tendency to sink due to overweight is shown by the following 

 comparison of sinking rates of active and narcotized specimens 

 of same specific gravity under similar conditions: 



