26 Bulletin Wisconsin Natural History Society [Vol. 11, Nos. 1-2 
and an alga (Hildenbrandtia) ; on right side anterior to legs, a 
Crepridula plana with its eggs; on legs, Spirorbis and a small 
Crepridula ; under abdomen, three Mytili (one large, two small). 
But we are perhaps dwelling over long on the relations of cer- 
tain sessile animals to others. Let us pass on to consider the 
marked sedentary character of the animals on the rock beaches. 
Not only were many of the animals permanently attached, but 
a number of those which had well developed powers of locomotion 
often remained for long periods of time fixed in one spot. Asterias, 
Littorina, crabs and ribbon-fish ( Pholis ) migrated up and down 
with the tides more or less, but Acmaea, Strongylocentrotus and 
Metridium sometimes remained for at least a fortnight in one 
22 23 
Fig. 22. Ventral View of a Cancer borealis 
that bore the following fauna: On the dorsal surface of carapace, Spirorbis 
spirorbis (and an alga, Hildenbrandtia ) ; on left side anterior to legs a Cre- 
pridula plana with its eggs; on right side, a small Mytilus edulis; on legs, 
Spirorbis and a small Crepridula ,* on swimmerets — three Mytili (one large, 
two small). 
Fig. 23. Hydractinia polyclina on a Littorina Shell Occupied by a 
Hermit Crab, and on a Balanus. 
spot. Several different areas in the bottoms of pools were mapped 
and observed for two weeks to determine how much the animals 
moved from time to time. Most Stryonglocentroti moved a little 
every time the tide covered them, though some were quiet two or 
three days, and one individual that was imbedded in a sort of 
a nest in a sponge ( Cliona ) (fig. 24) did not move for two weeks. 
Acmaea usually moved a little from time to time but few went 
out of the areas watched (5 to 6 ft.) during the period of obser- 
vation; only one individual retained the same place and position 
for more than seven days. No “scars” such as have been de- 
scribed for Patella and other molluscs (Willcox, ’05) were observed. 
