Why isn't a frog a reptile?

A Frog is an amphibian and amphibians and reptiles belong to different classes for a number of physiologic and developmental reasons.

AMPHIBIANS:

• Cold-blooded
• Smooth scale-less skin that needs to be moist.
• Produce soft, jelly-like eggs in moist/aquatic environments.
• Primitive three-chambered heart (single ventricle with two atrial arches); gills early in life, lungs as adults.
• two life stages (aquatic with gills and terrestrial with lungs)

REPTILES:

• Cold-blooded
• Scales made of keratin (protein) with rough skin underneath
• Produce semi-hard, leathery shelled eggs on land.
• Three-chambered heart, and lungs.
• One life stage (no metamorphosis)