The animal in Figure 42.1 may look like a creature from a
science fiction film, but it’s actually an axolotl, a salaman-
der native to shallow ponds in central Mexico. The feathery
red appendages jutting out from the head of this albino
adult are gills. Although external gills are uncommon in
adult animals, they help satisfy the need shared by all ani-
mals to exchange substances with their environment.