THE DISTRIBUTION OF ANIMALS 447 



other echinoderms abound in almost every sea; the 

 starfishes chiefly along the shore, the sea urchins in 

 the Littoral zone, and the sea slugs around coral reefs. 

 Worms are found in all parts of the world, in sea, fresh 

 water, and earth. They are most plentiful in the muddy 

 or sandy bottoms of shallow seas. Living brachiopods, 

 though few in number, occur in tropical, temperate, and 

 arctic seas, and from the shore to great depths. Poly- 

 zoa have both salt and fresh water forms, and annelids 

 include land forms, as the earthworm and some leeches. 



Mollusks have a world-wide distribution over land and 

 sea. The land forms are restricted by climate and food, 

 the marine by shallows or depths, by cold currents, by 

 a sandy, gravelly, or muddy bottom. The bivalves are 

 also found on every coast and in every climate, as well 

 as in rivers and lakes, but do not flourish at the depth 

 of much more than two hundred fathoms. The fresh- 

 water mussels are more numerous in the United States 

 than in Europe, and west of the Alleghanies than east. 

 The seashells along the Pacific coast of America are 

 unlike those of the Atlantic, and are arranged in five 

 distinct groups : Aleutian, Californian, Panamic, Peru- 

 vian, and Magellanic. On the Atlantic coast, Cape Cod 

 and Cape Hatteras separate distinct provinces. Of 

 land snails, Helix has an almost universal range, but is 

 characteristic of North America, as Bulimus is of South 

 America, and Achalina of Africa. The Old World and 

 America have no species in common, except a few in 

 the extreme north. 



The limits of insects are determined by temperature 

 and vegetation, by oceans and mountains. There is an 

 insect fauna for each continent, and zone, and altitude. 

 The insects near the snow line on the sides of mountains 

 in the temperate region are similar to those in polar 

 lands. The insects on our Pacific slope resemble those 



