NATURAL HISTORY OF ONCHIDIUM 449 
The following notes of one emergence of the Onchidia inhabit- 
ing a section of Little Agar's Island will give a sufficient general 
idea of the phenomenon, which we have repeatedly observed: 
July 6, 1914- A cloudy, bvit fairly bright day. Low tide at 11:30 
A.M. 
9:57 A.M. A few (4 or 5) Onchidia 'out,' others "in process of emer- 
gence. 
10:02 About a dozen out. Many at once begin to climb straight 
upward. Others wander in devious paths; if on a flat rock- 
shelf, may start toward the water, but do not actually go 
downward. 
10:12 In one cove on S. E. side of island 24 individuals seen. Over 
100 seen on flat rocks at opposite side of the cove, to the west- 
ward; at 10:02, only 1 or 2 were to be seen here. 
10: 17 A few stragglers are still appearing. 
10:32 A few (those which were the first to appear) are beginning to 
retire. 
10:37 Some Onchidia from nests lower down than those previously 
concerned are beginning to come out. For these, the tide is 
as much lower than the nest as it was in the case of those 
first appearing. 
11:02 Excepting a few, the Onchidia seen out at 10:12 have all 
returned. At 11:09, only 3 of this first group remained out. 
11:42 A community seen coming out at 10:30 has now completely 
retired. 
12:20 p.m. About 10 Onchidia, as nearly as could be determined, 
are now to be seen on the entire island. 
The foregoing record illustrates some of the general features of 
the appearance of the Onchidia upon the rocks and of their 
return to their nests. Several further records may be cited which 
exhibit the remarkable synchronous character of the return to 
the nest on the part of the different individuals of a colony, even 
when these individuals may be separated from one another by 
a relatively considerable distance. These records are typical 
of the many observations made upon this point, and no facts 
discordant with them have ever been encountered. 
July 2, 1914. 
9:30 a.m. a bright sunny day. Only a very few animals to be seen 
on Little Agar's Island. Low tide occurred at 8:00. [Doubt- 
• less the few animals visible were late comers from nests near 
low-tide level, but no note was kept of this.] 
9:40 On a rock 3 Onchidia, in a group, were seen creeping back and 
forth; one of these wandered two feet away from the others. 
