Actually they all look different, even two of the same specie have differences. It depends on how close you want to look.
However aquatic animals have all evolved because of their adaptability and those adapting features look similar.
The concept that explains the presence of mitochondria in cells is the endosymbiotic theory. Mitochondria in cells derived from the extra embryonic membranes.
This would be an expository essay, which explains a concept in detail.
The Concept of The Water cycle.
Theory
a
Obligations
OBLIGATION
A theory. (And sometimes called a law.)
how variations occur within a species
This would be an expository essay, which explains a concept in detail.
A SCIENTIFIC THEORY is a well-tested concept that explains a wide range of observation. (IF YOU ARE USING THIS TO CHEAT ON YOUR SCIENCE HOMEWORK, THEN YOU'RE NOT ALONE) lol
It is called a Scientific Theory.