196 University of California Publications in Zoology \Vou.9 
From the results so far obtained it appears that as regards 
up-and-down migration, these animals behave quite differently 
from the chaetognaths. For example, Calanus finmarchicus, one 
of the most abundant species, comes to the surface in greatest 
numbers between 7 and 8 p.m. (during June and July) and 
begins to descend about midnight, appearing to be nearly gone 
by four o’clock in the morning. No second return to the surface 
occurs the same day. Dr. Esterly points out that this confirms 
the conclusions reached by several other observers that some 
other factor or factors than light must come into play in deter- 
mining the movements. What these are remains to be ascer- 
tained. The animals descend to 200 fathoms in great numbers, 
but do not seem to go much deeper. As would be expected, it is 
during the day that they are found in the extreme depths. 
Another species, Hucalanus elongatus, ‘‘does not exhibit any 
well-marked diurnal migration,’’ and though very abundant at 
from 200 to 250 fathoms is never abundant on the surface. No 
cause has been definitely discovered for the failure of this species 
to make the customary daily excursions, but Dr. Esterly calls 
attention to the fact that the animals are particularly trans- 
parent, more so perhaps than those of any other species, and 
suggests that this may partially explain their peculiarity of habit. 
The larvae of the enteropneusts or “‘intestine-gilled’’ worms 
are the only other animals upon which anything has been pub- 
lished from this standpoint. Ritter and Davis (1904) point out 
that one species, Tornaria ritteri, almost never occurs on the 
surface, but at certain times and certain localities occurs in con- 
siderable abundance at depths from 25 to 100 fathoms. This 
conclusion was based on work done before closing-nets were used, 
and was not founded on a very great amount of data statistically 
treated. It nevertheless seems reliable. Although a quantitative 
study of the vertical distribution of the animals in relation to 
daylight was not made, laboratory experiments were performed 
to determine whether or not they react to hght. The results were 
negative; and no explanation other than the structural and 
functional constitution of the larvae were found for the phenom- 
enon of a pelagie period in the life of the species. Interesting 
relations were made out between stage of development, bulk, 
specific gravity, and power of swimming of the larvae. 
