1913] 
Pearse: Fauna of the Rock Beaches 
25 
Of the small ubiquitous motile animals the amphipods, isopods, 
and worms may be cited as examples. There are protected by 
their ability to hide under sea weeds or in crevices when the 
tide recedes. They occur in great abundance and become active 
when the ocean periodical^ covers them. 
The large, hard-shelled crabs which migrate back and forth 
from deeper to shallow water bring up many attached organisms 
on the shore which are characteristic of deeper water. Crepridula 
is abundant on shells and stones in deep water but was observed 
in shallow water only on Limulus (fig. 20) , in the shells of Pagurus 
(fig. 21a) and on Cancer (fig. 22). Spirobis was found only on 
Fucus and Laminaria in open water, in Pagurus shells (fig. 21 c), 
and on crabs. Hvdractinia occurred in the most exposed localities 
a b c 
Fig. 21. a, Pagurus acadianus in a Polyneces heros Shell With Cre- 
pridula plana; b, Acmaea testudinalis attached to Littorina litorea; c, Pagu- 
rus acadianus in a Polyneces heros Shell with Spirorbis spirorbis. 
on rock, algae, and barnacles, but also coated many Pagurus shells. 
Hydroides was never seen on the beaches except on the carapace 
of Cancer borealis. Attachment to a moving object is probably 
of considerable advantage to these small animals, yet, though 
such association has come to be quite constant in some cases 
(e.g., Pagurus and Hydractinia) , their association with particular 
large animals is usually more or less accidental. 
Considering the frequency with which small sessile animals occur 
attached to larger forms it is not hard to imagine how a barnacle 
might have become modified into a Sacculina-like animal or how 
a sessile mollusc might have been transformed into a parasite. 
One Cancer borealis (fig. 22) captured at Station 8 (fig. 1) bore 
the following fauna: on dorsal surface of carapace, Spirorbis 
