Q.What is the difference between direct and indirect development?

Direct and indirect development differ in whether a larval stage comes between the young and the adult:

• Direct development means the young that hatches or is born already resembles the adult, with no distinct intermediate stage. It simply grows into the adult. Bony fishes and reptiles show direct development. …

After an egg hatches or a young one is born, it may reach the adult form in one of two ways, and the difference lies in whether a distinct larval stage is involved.

• Direct development. The young resembles the adult from the start, without a separate larval phase, and grows steadily into the adult without any marked change in body form. Among the vertebrates, bony fishes show direct development, and so do reptiles — their young are miniature versions of the adult. …

Method: The life-cycle timeline comparison

Draw out the life history of a bony fish and an amphibian as two parallel timelines from egg to adult, and read the difference straight off the shape of each timeline.

Timeline 1 — a bony fish (direct development): Egg → hatchling that already has fins, gills and the general fish body shape → simply grows larger over time → adult. The timeline is a single straight line: one body plan from hatching onward, just scaled up.

Timeline 2 — an amphibian, such as a frog (indirect development): Egg → hatches into a tadpole, an aquatic, gill-breathing, tailed, limbless larva that looks nothing like a frog → the tadpole undergoes metamorphosis (loses its tail, grows four limbs, develops lungs) → adult frog. This timeline has a visible kink in it: the larval body plan is a genuinely different shape from the adult, and a transformation step is needed to bridge the two. …