What external and internal features indicate that the starfish is organized in a pattern of fives?

Starfish, also known as sea stars, exhibit a distinct five-fold symmetry, which is evident in both their external and internal features:

External Features:

* Five arms: The most obvious feature is their five arms radiating from a central disc. This gives them their characteristic star shape.

* Five ambulacral grooves: Running along the underside of each arm is a groove called an ambulacral groove. These grooves house the tube feet, which are used for movement, feeding, and sensory perception.

* Five rows of tube feet: Each ambulacral groove contains a row of tube feet, resulting in a total of five rows extending from the central disc.

* Five madreporites: These small, sieve-like structures on the aboral (upper) surface of the central disc act as the entry point for seawater into the water vascular system. Sea stars typically have five madreporites.

* Five gonads: The reproductive organs, or gonads, are also arranged in fives, with one in each arm.

Internal Features:

* Five radial canals: The water vascular system, responsible for locomotion and feeding, branches out from the ring canal in the central disc into five radial canals, one running down the center of each arm.

* Five digestive glands: These glands, located in each arm, help digest food and absorb nutrients.

* Five nerve cords: The nervous system is organized with five nerve cords radiating from a central ring in the central disc.

This five-fold symmetry is known as pentamerism. It's a key characteristic of echinoderms, the phylum to which starfish belong. This radial symmetry is advantageous for starfish as it allows them to move and feed in all directions without having a distinct head.