Handbook of Paleontology 91 



alternating marine and terrestrial conditions. In this 

 adaptation rock pools and algae play an important part. 

 The animals may be grouped into two divisions, non- 

 sessile and sessile or sedentary forms. The nonsessile 

 forms have the advantage of being able to retreat with 

 the tide or to retire to tide pools in the case of forms 

 inhabiting rocky shores. On sandy shores considerable 

 moisture is held by the sand and certain forms keep in 

 touch with moisture by burrowing in this medium. A 

 large amount of moisture is also retained by seaweeds 

 and under stones, and many animals take advantage of 

 this. Crevices in rocks also afford another means of 

 keeping in touch with moisture. The adaptation of non- 

 sessile forms is therefore an adaptation of behavior. 



The problem in relation to the tide factor is much more 

 difficult for sedentary animals and their response is in 

 some suitable modification in structure which aids in re- 

 sistance to drying out during the hours when the tide is 

 out. One of the best examples of this is the Acorn 

 Barnacle (Balanus). These barnacles are found living 

 on rocks so far above ordinary high water that they re- 

 main dry for days at a time, and experiments have shown 

 that they can live for 44 days out of water. The cup in 

 which the animal lives is covered by a valvular roof com- 

 posed of four accurately fitting plates. As one walks 

 over barnacle-covered rocks at low tide, one may hear a 

 clicking sound due to the complete closing of these valves. 

 At low tide the barnacle imprisons a small bubble of air 

 between the tips of the valves and makes use of it for 

 breathing purposes; if disturbed, the valves are com- 

 pletely closed and the air bubble released. Gastropods 

 or snails use the operculum to serve the same purpose and 

 tube-building polychaete worms have a terminal enlarge- 



