362 COMPARATIVE ZOOLOGY. 
mostly obtained from the Grecian Archipelago and Baha- 
mas. Corals abound throughout the Indian Ocean and 
Polynesia, east coast of Africa, Red Sea and Persian 
Gulf, West Indies and around Florida. True Crinoids 
are found only in the Caribbean Sea and on the coast of 
Norway. The other Echinoderms abound in almost ev- 
ery sea, the Star-fishes chiefly along the shore, the Sea- 
urchins in the Laminarian zone, and the Sea-slugs around 
coral-reefs, 
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 mud bottom. Living Brachiopods, 
though few in number, occur in tropical, temperate, and 
arctic seas, and from the shore to the greatest depths. 
The rest of 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 200 fathoms. 
The fresh-water Mussels are more numerous in the United 
States than in Europe, and west of the Alleghanies than 
east. The sea-shells along the Pacific coast of America 
are unlike those of the Atlantic, and are arranged in five 
distinct groups——Aleutian, Californian , Panamic, Peruvian, 
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 
Achatina 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 Pacitic slope resemble those of Europe, 
