The animals of the seashore include those species confined to or spreading into the intertidal zone, that narrow fringe of the coastline over which the tide rises and falls twice daily. The tides give a unique character to the ecology of the shore. A sequence of highly adapted animals and plants, of many different classes and life forms, girdles the whole land mass. Shores are divided first and most fundamentally into the two categories, hard and soft. Few species overlap between the two and the conditions of life in the two habitats are utterly different.