INTERNAL ANATOMY

Although G. watsoni bears a tough and durable exterior, its internal anatomy exhibits complex yet delicate biological machinery. The internal anatomy of an asteroid supports a unique water vascular system as well as digestive and reproductive systems.

The water vascular system is unique to echinoderms. The water vascular system consists of a coelomic compartment made up of a set of canals and tube feet that form a specialised hydraulic system (Ferguson 1995; Hickman et al. 2008). In asteroids, the water vascular system is specialised

to aid in locomotion, feeding, respiration and excretion. The madreporite, being an asymmetrical ‘cap’ to the water vascular system

on the external, aboral surface of a sea star has small pores that open up to the water outside.  The madreporite is connected to the internal stone and ring canals around the mouth (Figure 1), which are then connected to radial and lateral canals in the arms (Figure 1). These internal canals primarily function as a means of circulation and fluid-volume maintenance for hydraulic function (Ruppert et al. 2004). The tube feet of the sea star are connected to the lateral canals of the water vascular system that are able to mov

e and create movement through the contraction and pumping of water through the ampulla (Figure 1). It is through the hydraulics and movement of water through the system that the sea star is able to manoeuvre, feed, respire and excrete.

Nervous System

In asteroids, three component systems comprise the nervous system

s that are present on various levels in the disc and arms. Firstly, the ectoneural  (sensory) system consists of the nerve ring that surrounds the mouth and radial nerves in each arm which are known to coordinate the tube feet (Ruppert et al. 2004). Secondly, the hyponeural (motor) system that is located above the ectoneural system and finally the aboral system that consists of the ring around the anus and the radial nerves (Hickman et al. 2008). These systems are connected by the epidermal nerve plex

us that acts as a nerve net and coordinates in response to physical stimuli. As sensory organs are not highly developed in asteroids, sea stars may rely on pedicelliarie and tube feet for sensory reception and directional movement.

Digestive System

The digestive system of asteroids is both interesting and unique. The mouth is located on the oral side that leads to a short oesophagus and large stomach located within the central disc. The seastar feeds by everting the cardiac (lower) part of its stomach through the mouth and is then sucked back in. Thus, digestion often occurs externa

lly to the body by muscle contractions and digestive enzymes that engulf prey. The pyloric (upper) part of the stomach connects to the pyloric caeca or digestive glands that aid in nutrient storage (Figure 2). Digestion occurs by both extracellular and intracellular means (Hickman et al. 2008). Similarly, a shorter intestine connects to the pyloric stomach where intestinal ceca are present. Waste is excreted out a very inconspicuous anus, although some asteroids lack an anus altogether.