two separate atria.

an incompletely divided ventricle.

two separate ventricles.

systemic and pulmonary circulation.

cutaneous respiration.

The viscosity of the water results in substantial drag

The negative buoyancy of most animals mean they have to struggle to keep moving in a water column

The effect of gravity on a swimming animal

The lack of a solid surface to push against (like soil or rock)

The regular arrangement of hydroxyapatite crystals within the extracellular matrix.

Spongy bone tissue.

A matrix of hydroxyapatite mineral crystals and protein collagen.

The system of Haversian canals.