1911] Michael: Chaetognatha of the San Diego Region. 121 



cessation of activity or quiescence of the animals which would 

 cause them to sink, since their specific gravity is greater than 

 that of water, or more probably to the usual relatively high 

 surface temperature (see p. 132). 



It is obvious that when light increases in intensity the water 

 nearest the surface is first affected, while when it decreases the 

 reverse is true. During morning twilight, therefore, we should 

 expect the species to ascend first from 4-6 fathoms, later from 

 7-12 fathoms, and still later from 15-20 fathoms. During even- 

 ing twilight, however, we should expect migrations first from 

 deeper water. Again, after the morning light becomes too in- 

 tense, the animals ought to leave the surface first and the suc- 

 ceeding depths later. While our data are inconclusive owing to 

 absence of closing net hauls during many periods of the day, 

 still, when we endeavor to ascertain where the increasing number 

 of specimens on the surface come from and where those leaving 

 the surface go to, the following few facts extracted from table 

 38 are significant. 



1. While an increase from 24 to 338 specimens per hour on 

 the surface took place from midnight to 4-6 a.m. the number in 

 4-6 fathoms decreased from 36 to 3. Evidence of what hap- 

 pened in 7-12 fathoms is lacking, but in 15-20 fathoms the 

 number increased from 36 to 96. This suggests that the inten- 

 sity of twilight between 4 and 6 a.m. was sufficient to cause 

 upward migrations from 4-6 but not from 15-20 fathoms. 



2. While the number of specimens per hour on the surface 

 increased from 338 between 4 and 6 a.m. to a morning maximum 

 of 519 between 6 and 8 a.m., the number in 4-6 fathoms in- 

 creased from 3 to 80, while that in 7-12 fathoms decreased from 

 41 to 31. and that in 15-20 fathoms from 96 to 27. This indi- 

 cates that the intensity of light after sunrise was sufificient to 

 cause upward migrations from 7-12 and 15-20 fathoms, some 

 specimens reaching the surface while others accumulated in 4-6 

 fathoms. 



3. Combined with a decrease from 519 specimens per hour 

 on the surface between 6 and 8 a.m. to 169 between 8 and 10 

 a.m. and a decrease in 7-12 fathoms from 31 to 0, is an increase 



