Filtering 125 mL of plasma every minute is a big job. To manage it, your kidneys have many nephrons -- over one million in each kidney.
To make the most of each nephron, the tubules are long and coiled up, so that the urine passes through a longer distance before it goes out and the kidney has more time to modify it through absorbtion or secretion.
These twisted tubules are called convoluted tubules.
In an aquatic animal with no concerns about water balance, nephrons only need a filter and some convoluted tubules. This is the kind of nephron you'd find in some fishes, or in an early human embryo.
But this kind of nephron lets a lot of water leak out. An animal living on land wouldn't be able to survive with this kind of nephron; it would lose too much fluid in its urine and die of dehydration.