Have you ever wondered why starfish have five arms? Or why sand dollars have five openings in their carapace arrayed in the shape of the star? Well, it's because they are members of the Phylum Echinodermata, and all members of this phylum are pentaradially symmetrical1. Other members of this phyla who share this trait are the sea lillies (Class Crinoidea), the starfish (Asteroidea), the brittle stars (Ophiuroidea), the sea urchins (Echinoidea) and the sea cucumbers (Holothuroidea).

Pentaradial symmetry is defined as having five identical parts (penta) arrayed around a central axis (radial). As such, each animal has five sections which are identical in construction and appearance. The best example of this body plan is the starfish, which has its five arms extending from its central axis. In each arm, identical respiratory, reproductive, nervous and locomotory systems. Even the stomach, which is centrally located, has the shape of a pentagram.

Perhaps imagining a pentaradially symmetric human helps to illustrate the point and see the world from the sea cucumber's point of view. Such a human would have a head shaped more or less like a pentagram, with one eye in each section. The mouth would likely be located at the apex of the head, making kissing difficult to say the least. This human would have five arms and five legs, and be able to break dance in heretofore unimagined ways. To simplify the digestive system, this person would likely have a centrally located stomach and, if constrained evolutionarily to be like the Echinoderms, would not have an anus or bowel. Instead, after digestion the waste products would simply be disposed of through the top of the head by stomach eversion. Finally, if this person were female they would likely have one or two breasts on each segment (a Russ Meyers fantasy if I've ever heard of one), or if male would have five penises, like most Anime villains. Despite these remarkable reproductive systems, the general body plan would probably be best adapted to external fertilization: imagine an equal-opportunity bukkake fest.

On second thought, I'm not sure that that image helps anyone at all.

1 Say that five times quickly without paralyzing your tongue and you win a sea urchin.

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