CH. i] HABITATS OF ANIMALS. 3 



inhabits ; the mud fish of Africa and America has lungs 

 as well as gills, and can suffer with equanimity the drying 

 up of the rivers in which it lives. Eels are said to move 

 from one pool to another across the intervening grass. 

 The curious little fish PeriopMhalmus voluntarily leaves 

 the sea and hops along its margin on the look-out for the 

 Mollusc Onchidiwm upon which it largely feeds. Among 

 Invertebrates there are more purely aquatic groups ; and 

 these are exclusively marine. No Ascidian, no starfish, 

 brittle star, sea urchin, or sea lily has as yet been met 

 with except in the sea. Sponges and Coelenterates are 

 without exception aquatic and for the most part marine, 

 their delicate organisation not being able to withstand 

 continued dryness. But these are positively the only large 

 groups which are purely aquatic. Many others are prin- 

 cipally aquatic, such as the Flat worms, with terrestrial 

 allies in the land Planarians, the Crustacea, the Annelids 

 and some others. 



Not only can we draw these broad distinctions between 

 the habitats of different animals, finding one to be terres- 

 trial, another aquatic and a third amphibious ; we can also 

 assign to each a definite place upon the land or in the 

 waters. The Indian Ocean is frequented by creatures 

 which are unknown in the colder waters of the North Sea ; 

 the Mississippi has alligators which the Thames, for ex- 

 ample, has not. It is a matter of common knowledge that 

 the tiger is restricted to Asia and the puma to South and 

 certain parts of North America ; the elephant is unknown 

 beyond the old world, and it has even there a limited area 

 of range. It would be as surprising to meet with an 



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