1913] 
Pearse: Fauna of the Rock Beaches 
27 
Metridium, though it has well developed powers of locomotion 
(MacClendon, ’06) did not move appreciably; two colonies near 
the low water mark were mapped, but none could positively be 
said to have moved after two weeks, though eightv-four indi- 
viduals were under observation. 
It has been said that the rock beach fauna is a hardy one, and 
indeed it is to be expected that animals which are subjected to 
such variable conditions would develop considerable resistance. 
At Nahant the tides rise and fall through a vertical distance 
of about eight feet. Those animals which do not migrate are 
alternately subjected to submergence under the cool waters of 
Fig. 24. Strongylocentrotus drobachiensis in a “Nest” formed by the 
Growth of the Sponge, Cliona sulphured around it. 
the ocean and to dessication under the direct rays of the sun. 
Fucus and Ascophyllum often become so dry that they crackle and 
break when touched, but they are apparently uninjured by such 
dessication and soon become pliable when wet by the rising water. 
A calcareous exoskeleton is characteristic of many inhabitants of 
the beach and doubtless affords adequate protection for their 
soft bodies against dessication and the extreme^ of temperature. 
Balanus and Mytilus seem to suffer no inconvenience from being 
baked by the sun, for they begin to feed within a minute after 
they are covered by the cool water of the rising ocean. Littorina 
has a similar resistance, and is influenced by the surface film as 
Haseman (Tl) well demonstrated. 
