140 How Animals Changed 



Acclimatizations may be concerned with resistance, toleration, 

 enlarged or restricted ranges of adjustment, changing tropisms and 

 habits, or other things. They usually involve complex groups of 

 activities, but the net results permit animals to live in a changed 

 environment. 



Parasites 



In the ocean many animals are associated with others as com- 

 mensals, symbionts, or parasites, but such relations are not usually 

 carried over into fresh-water and land habitats. Parasites show a 

 general tendency to become specialists in their host relations. Many 

 are restricted to a single species, genus, or family, though there are 

 also numerous wide-ranging types. Parasites are often used as in- 

 dicators of the past history and relationships of their hosts (Met- 

 calf, 1929) . 



Parasites that occur in and out of the water along shore are of 

 special interest in connection with aboceanic migrations. In such 

 widely separated localities as the coasts of China, Japan, and Dry 

 Tortugas (Pearse, 1929, 1930a, 1931) the land crabs along shore 

 carry more parasites than crabs in the ocean. Some of the terrestrial 

 parasites (mites) evidently were acquired on land, but even such 

 types as certain species of parasitic copepods (Wilson, 1913; 

 Pearse, 1930b) and commensal vorticellids (Pearse, 1930a, 1932c) 

 are seldom found on aquatic crustaceans and are quite common on 

 terrestrial types because the bloods of such hosts are more stable 

 and nutritious than those of their more primitive relatives in the 

 ocean, but the consistent presence of others is not so easy to explain. 

 In Japan a rather careful survey of the parasites of eight species 

 of salamanders which ranged from aquatic to terrestrial types 

 showed some peculiar anomalies which were apparently due more to 

 host specificities than to habitat relations (Pearse, 1932f ) . Among 

 protozoans: Trypanosoma and Balantidium were found only in an 

 aquatic newt; Trichodina only in the bladder of a land salamander 



