What is the ambulacral groove on a starfish?

The ambulacral groove is a deep linear depression located in the ambulacrum in some echinoderms such as starfish. The ambulacrum is an area in echinoderms that is marked by calcitic plates and located over the radial canals of the internal water-vascular system.

In the starfish, the ambulacral groove is located on the oral side, which is the underside of its body, and extends from the mouth to each ray of the arm. Most echinoderms have ambulacral areas that exist in fives or multiples of five. Ambulacral parts may include plates, suckers, spines and ossicles. Ambulacra have structures known as interambulacra that are located between them.