Handbook of Paleontology 133 



and methods of attachment which make them admir- 

 ably suited to withstand the force of the waves. They, 

 moreover, succumb in the order given. The Acorn 

 Barnacle is the first to cover a clean, exposed rock 

 surface and the last to leave it. Next to the above 

 forms come the Periwinkles and the Rock Snails. At- 

 tached coelenterates, sea anemones and hydroids, are 

 also well suited to a life on the rocks. The hard-shelled 

 animals, as the crabs, are able to withstand consider- 

 able beating by the waves and by migrating downward 

 with the tide they escape exposure to the sun. The 

 small, ubiquitous, motile animals, including small crus- 

 taceans and worms, hide under seaweeds and in crev- 

 ices when the tide recedes and thus secure protection. 

 They occur in great abundance and become very ac- 

 tive when the tide is in. Many attached organisms char- 

 acteristic of deeper waters are brought in to the shore 

 by the large, hard-shelled crabs that migrate back and 

 forth. The rock beach fauna must be, and is, a hardy 

 one. The factors that count are fixation, ability to cap- 

 ture food and ability to withstand exposure and changes 

 in temperature. The animals that show the greatest de- 

 velopment of the above characteristics have the widest 

 distribution. The Acorn Barnacle is everywhere, there- 

 fore, because it has a firm fixation, the ability to capture 

 food anywhere that seawater comes, and a remarkable 

 resistance to changes in temperature. The exposure and 

 variation in the barnacle zone are greater than anywhere 

 else on the beach. Seaweeds are absent from this zone, 

 but the Edible Mussel (Mytilus edulis) is found in crev- 

 ices right to the top of the zone. The Periwinkle or 

 Rough Winkle (Littorina rudis) often occurs on bare 

 rocks above the barnacles. This form is on its way 



