Final answer:
Amphibians spend part of their life cycle in water and part on land, starting as water-breathing larvae and metamorphosing into adults capable of living on land.
Step-by-step explanation:
The group of organisms that spend half of their life cycle in water and the other half on land are amphibians. Amphibians, like frogs, salamanders, and caecilians, begin their lives as larvae in water where they breathe using gills.
Upon maturing, they undergo a process called metamorphosis, during which they develop lungs and limbs for terrestrial life, although they still need to remain close to water for moisture and reproduction. Their dual life spans both aquatic and terrestrial habitats, with their skin also playing a role in respiration alongside the lungs.