The lower surface of a starfish is called the oral surface or ventral surface. Another word for ventral is "abdominal." The mouth of the starfish is located in the middle of the oral surface.
Starfish belong to the phylum Echinodermata, which means they are invertebrates. Their arms, which typically number between five and 40, have little tube feet that aid movement and feeding. On the oral surface, a groove called an ambulacral groove stretches the length of each of the arms. An eye is located at the end of each arm. The upper side of the starfish is known as the dorsal surface or the aboral surface.