EXTERNAL MORPHOLOGY

The exterior of asteroids is often hard and tough in order to protect the delicate organs beneath. The external morphology consists of a central disc that bears five (sometimes more) arms (Figure 1). The flat, flexible body of the sea star consist of ciliated, dark grey pigmented epidermis (Hickman et al. 2008). The tough and spiny body wall is comprised of calcium and magnesium carbonate ossicles that provide a form of defence for the seastar (Bryan et al. 1997). The small calcareous ossicles that form the mesodermal endoskeleton lie beneath the epidermis, adhered by connective tissue (Hickman et al. 2008). This contributes to the external strength of the endoskeleton, provides structural support and may often become soft to allow evasion of predators (Bryan et al. 1997). Exhibiting an oral-aboral axis, the mouth is located on the oral side beneath the central disc that is surrounded by a soft peristomial membrane (Figure 2). Running from the mouth to the end of each arm is the ambulacrum, which is an opening or groove from which the tube feet emerge (Figure 2). Pincer-like pedicelliariae are also present on the external surface that act as tactile sensors to physical stimuli (Lambert et al. 1984) and to keep the body surface free of debris or sedim

ent (Hickman et al. 2008) (Figure 3) .