STUDIES IN MARINE ECOLOGY. 215 



The animals mentioned may be displaced somewhat from the 

 order given but as one looks down the outer piles at Vineyard 

 Haven four distinct bands can easily be recognized : The Balanus 

 band well above the low water level; the Mytilus band near 

 low water, the Amarcecium-Tubularia band which shows as a 

 more brilliantly colored region and finally the dull brown of the 

 Metridium band which extends well down toward the bottom of 

 the pilings. 



Apart from the obvious relations in the upper portion where 

 independence of water is the controlling factor, the vertical 

 distribution may be in part the effect of light differences partic- 

 ularly on the free swimming larvae. Grave ('20) has found that 

 Amar cerium tadpoles, for example, are first positive but soon 

 become negative to light. In part the position may be a response 

 to gravity for the same tadpoles are at first negative in their 

 reaction to gravity and later become positive. When both light 

 and gravity act as they would in nature, the tendency of the 

 tadpoles to settle to the bottom of the experimental cylinder was 

 most marked and occurred in 85 per cent, of the cases. The 

 banding may also be controlled in part by reactions to pressure 

 and to conditions in the water that could not be detected by 

 the measurements made. On the whole Grave is inclined to 

 attribute the distribution of Amarcecium colonies to the effect of 

 gravity during the active, free-swimming life of the tadpoles. 



c. Rock Associations. — There are two distinct types of animal 

 associations to be found in the region dominated by rocks. 

 First, there are the animals and plants attached to the rocks and 

 rockweeds or living among those so attached and, second, there 

 are the animals living in the substratum under and sometimes 

 between the rocks. These last are closely similar to the animals 

 of the mud or sand flats near which the rocks are located. No 

 attempt has been made to get water samples exactly characteristic 

 of the second community, since in this work the main attention 

 was focused on the animals of the rocks. 



Two animal associations of the rocks proper are easily recog- 

 nized. On the exposed rocks, where there is little rockweed, 

 the animals are mainly found under the edges of the rocks out 

 of the direct sweep of the waves. In protected regions, the rocks 

 are covered with the rockweeds, Ascophyllum and Fucus. Here 



