•Water vascular system: a collection of hydrostatic structures used in association with their fluid filled cavities (coelom) and tube feet to help circulate water through their bodies and aid in movement across the substrate. They also utilize a special opening located on the anterior side of the central disc called a madreporite, which is used to intake water for the water vascular system.

Acanthaster planci tube feet.
Image courtesy of Joseph Dougherty, M.D./

   •Spines: elongated spines for increased protection and also contains venoms such as saponins and plancitoxin I.
    •Ossicles: the spiny plates on the endoskeleton composed of calcium carbonate. They can combine to form a lattice which increases strength, reduces weight, and can even act as camouflage.

Acanthaster planci spines (left) and Acanthaster planci wrapped around coral (right).

    •Gas exchange: Acanthaster planci relies on diffusion across the surface of their body to exchange gas. This can be one reason why Acanthaster planci, as well as other starfish species, can grow so big to increase their surface area to allow for more gas exchange. Oxygen is acquired through the tube feet when water passes over them.
        -Tube feet: used in mobility (water vascular system) and gas exchange.
    •All starfish have the ability to regenerate lost limbs as long as the central disc remains intact.
    •Pedicellariae: these clamp-like structures are used to help keep debris, such as algae, off the surface of the starfish.

Image taken from Wikipedia. Author: Philippe Bourjon.

Excretory system: The mouth is located on the ventral side (bottom) of the starfish and the anus is located on the dorsal (top) side. Food is absorbed through the stomach by a process called eversion (see Nutrition). Acanthaster planci releases waste through the anus.

The dorsal side of a Crown-of-Thorns starfish (left) and another Crown-of-Thorns starfish (right) feeding on coral.

Sensory: Starfish lack a centralized brain but make up for it with clusters of nervous tissues known as ganglia located throughout the body. A cluster of ganglia can be seen on Acanthaster planci which appear to look like eyespots. These eyespots are used to detect changes in light and darkness.

Now take a look at what makes this species so dangerous: Venom