Conclusion 141 



that bred in water; and Opalina and Endamoeba in a land sala- 

 mander. Amphistomes, distomes, and Rhabdias were found only in 

 aquatic salamanders; Gorgoderina and cestode cysts were found 

 only in terrestrial salamanders. At the mouth of the Menam Chao 

 Phya in Siam the aquatic crabs had more parasites than those on 

 land, largely because two species of large portunids often had their 

 gill-cavities filled with barnacles (Pearse, 1933a) ; the land gobies 

 also had fewer and different types of parasites from those that lived 

 in the river itself (Pearse, 1933). The writer has spent several 

 years trying to discover significant differences between the parasites 

 of related aquatic and terrestrial animals. A few facts have been 

 discovered, but on the whole the results are as yet disappointing, 

 perhaps largely on account of host specificity. More work is needed. 



Conclusion 



Animals that have left the stability of the ocean for the greater 

 environmental instability that obtains to some extent in fresh water 

 and to a greater degree on land have been obliged to modify their 

 internal organizations to keep pace with external changes. Their 

 skins have grown more selective in permitting or preventing the pas- 

 sage of solutes and finally have been modified to conserve body 

 water and regulate temperature. Respiratory organs have changed 

 so that breathing has been transferred from external water-bathed 

 gills to internal, moist pulmonary cavities. Body fluids have changed 

 from salty solutions closely resembling sea water to hypertonic but 

 less saline, nutritious, viscid, gas-transporting media which maintain 

 standard and rather constant concentrations. Metabolism has come 

 under the control of many regulatory mechanisms and on the whole 

 proceeds more rapidly. Land animals generally live faster than 

 aquatic animals, partly because they have continual access to large 

 amounts of oxygen and partly because they live in a more change- 

 ful, and therefore a more stimulating, environment. Fresh-water 

 animals generally live faster than marine animals because they con- 



