Final answer:
The transverse (horizontal) plane is the plane that divides an animal into dorsal and ventral portions, separating it into upper and lower portions.
Step-by-step explanation:
A plane that divides an animal into dorsal and ventral portions is the transverse (horizontal) plane. This plane cuts through the animal, separating it into upper and lower portions. For example, if you imagine a line running from the animal's head to its tail, the transverse plane would be perpendicular to this line.