117 
From this table the following facts appear: 
1. That the volume taken at the top is in every case less 
than that taken at greater depths. Compare, for instance, 
Nos. 2, 3, 5 with Nos. 4, 8, 11. 
A comparison of Nos. 3, 5, 6, taken in the same locality 
on the same day, seems to show that the volume increases 
to a certain depthand then again diminishes. At the sur- 
face, or a few inches below, there is only .5 c.c. for each 
600 cubic feet of water; at 24feet, there are 7.2 c.c., and 
at 65 feet, 4.06 c.c. for the same volume of water. This 
result cannot, however, be regarded as final, since it rests 
on but one setof observations. Itis well known that many 
of the animals composing the plankton migrate toward the 
surface at night and seek greater depths during the day, 
and it may be that the difference between a cloudy and a 
sunny day produces a corresponding migration. 
~3. At the same depth, or about the same depth, the vol- 
ume taken varies with the locality. 
The observations refer to four localities: 
(a) At Frankfort, Michigan, about one and a half miles 
from land. 
(b) In the neighborhood of the Manitou and Fox Islands, 
between thirty and forty miles north of Frankfort. 
(c) At Ludington, Mich., about fifty miles south of 
Frankfort, and between two and four miles from shore. 
(d) The Detroit River, a few rods above Belle Isle Bridge. 
The first three localities cover a stretch of about eigltty 
miles along the east shore of Lake Michigan, near its 
northern end. 
A comparison of the region at Frankfort and the Mani- 
tous shows that at corresponding depths there is a larger 
volume of plankton than at Ludington. Thus, No. 1 at 
Frankfort, at a depth of 7.6 feet, shows 5 ¢.c. per 600 
cubic feet, and No. 10 at Ludington, at 6.75 feet, shows 
but 1.85 volumes. No. 3 at Manitou, at 24.5 feet, 
shows 7.2 volumes, while No. 9 at Ludington, at 13.5 feet, 
