1913 ] 
Pearse: Fauna of the Rock Beaches 
21 
“ roots” of the laminarians were many worms (Lepidonotus , 
Harmothoe, Phyllodoce, Nereis pelagica), and the brittle-star, 
Ophiopholis. 
6. Inter-relations of the Zones 
Zonation in the distribution of plants or animals means that 
the struggle for existence is so severe that each species is kept 
rather close to optimum conditions, either by the “ physical en- 
Fig. 16. Mytilus edulis, Balanus, and Cancer borealis in Chondrus Zone. 
vironment” or by competition. When conditions vary, zones will 
vary. In the rock beaches at Nahant we accordingly find par- 
ticular strata spreading when the conditions are favorable and 
“pinching out” when the struggle becomes too severe. The chief 
factors which cause zonal variations are: (1) degree of exposure 
to the open ocean, to the sun, or to winds; (2) the slope of the 
beach; (3) the character of the surface (smooth, rough, boulders, 
loose stones, etc.). 
Proximity to the open sea (with low comparatively uniform 
temperature and purer water) is a favorable factor for there is 
