Starfish are perhaps the most beautiful inverts among the aquaria world. This fellows are composed of a central disc from which arms sprout in pentaradial symmetry. Most starfish have five arms, however some have more or fewer; in fact some starfish can have different numbers of arms even within one species. The mouth is located underneath the starfish on the oral or ventral surface, while the anus is located on the top of the animal. The spiny upper surface covering the species is called the aboral or dorsal surface. On the aboral surface there is a structure called the madreporite, a small white spot located slightly off-center on the central disc which acts as a water filter and supplies the starfish's water vascular system with water to move. The body cavity also contains the water vascular system that operates the tube feet, and the circulatory system, also called the hemal system. Most species are generalist predators, some eating clams, and oysters; or any animal too slow to evade the attack. Other species may consume coral polyps, sponges or even suspended particles and planktons. Starfish are capable of both sexual and asexual reproduction. Individual starfish are male or female. Fertilization takes place externally, both male and female releasing their gametes into the environment. Resulting fertilized embryos form part of the zooplankton.