Metabolism 123 



related to rate of metabolism, as determined by temperature (Mac- 

 Arthur & Baillie, 1929) . 



The most active marine fishes have most haemoglobin in their 

 blood and carry on metabolism at a high rate; sluggish fishes are 

 better able to utilize oxygen when it occurs in low concentrations, 

 and to endure lack of oxygen (Hall, 1929) . Rate of metabolism 

 has apparently not been an important factor in the migration of 

 fishes from sea to fresh water or land, but it has been for many in- 

 vertebrates. For example, the respiratory rate of crabs varies more 

 or less with the salt concentration in the surrounding medium. This 

 happens because the rate of metabolism depends somewhat on the 

 amount of osmoregulation required (Schwabe^ 1933). Many ma- 

 rine invertebrates cannot increase their energy utilization in fresh 

 water and therefore perish. Some, however, are able to make tem- 

 porary or permanent adjustments to waters of low salt concentra- 

 tion. The kidneys of shrimp of the genus Palaemonetes and of 

 other crustaceans vary in structure according to the salinity of the 

 medium in which the animals live. The thyroid gland of vertebrates 

 produces a hormone, thyroxin, which increases metabolic rate. ''It 

 seems the business of the gland to maintain in the blood a certain 

 'definite though small concentration of an iodine compound. In 

 doing so it may, as has been suggested, maintain in the blood for the 

 use of the body the iodine content of the sea, and may thus have 

 made possible the evolution of the land animal" (Rogers, 1927) . 

 The temperature of a poikilothermic animal in any environment is 

 as a rule a little higher than that of its surroundings. Such animals 

 may raise their body temperatures a little by increasing their metab- 

 olism. Smith (1930) estimates that 15% of the metabolism of an 

 active Protopterus is expended in muscular movements. On the 

 whole, poikilothermic animals are at a disadvantage because many 

 of their activities must be suspended when temperatures fall. When 

 they leave aquatic habitats, where the high specific heat of water 

 makes rapid change in temperature infrequent, they are often con- 



