Stuffed Animals

     Starfish express pentamerism, that is, pentaradial symmetry. Most starfish have five rays or arms that radiate from a central disk, although several species frequently have six or more arms. At the end of each arm or ray, there is a microscopic eye, known as ocellus that helps the starfish to differentiate between light and dark, which is useful to see movement. The bodies of starfish are composed of ossicles, which are made of calcium carbonate. These form the endoskeleton, which takes on various forms that are externally expressed as a variety of structures, like spines and granules. The architecture and individual shape or form of these plates which occur in specific patterns or series, as well as their location are the source of morphological data that is used to classify the different groups within the Asteroidea. The terminology referring to body location in starfish anatomy is usually based in reference to the mouth to avoid an incorrect assumption of homology between the dorsal and ventral surfaces in other bilateral animals. Thus, the bottom surface is referred to as the oral or the actinal surface, whereas the top surface is referred to as the aboral or abactinal side.

     The madreporite is a structure that can be easily identified as a light-colored circle, located slightly off center on the central disk. This is a porous plate whose function is, at least in part, to provide additional water for the animal’s needs, including replenishing water to the vascular system. Something that is peculiar to starfish anatomy is what is known as pedicellariae. These are small bear-trap or valve-like structures that are seen in certain groups. These can occur all over the body or may occur in tufts at the base of each spine. These structures are thought to act as defense whereas others have been seen to aid in feeding. Some species even use these structures to capture and kill prey. Read more on interesting facts about starfish.

     Starfish Internal Anatomy

     Digestive System
     The mouth of a starfish is located on the underside of the body and it opens through a short oesophagus into first a cardiac stomach, which then leads into a second, pyloric stomach. Each arm of the starfish also contains two pyloric caeca, long hollow tubes branching outwards from the pyloric stomach. A short intestine runs from the upper surface of the pyloric stomach and opens at the anus in the center of the upper body. Waste is excreted through the anus on the aboral or abactinal side of the body. Starfish anatomy is such that it has the ability to hunt prey that are much larger than its mouth would otherwise allow, such as clams and oysters, arthropods, small fishes and mollusks.

     Nervous System
     Starfish have a rather complex nervous systems but lack a true central brain. As is common to all echinoderms, starfish have a network of interlacing nerves called a nerve plexus which lies within and below the skin. Although the echinoderms do not really have many well-defined sensory inputs, they are sensitive to touch, light, temperature, orientation, and the status of water around them. Many starfish also possess individual photoreceptor cells across their body and help them to respond to light even when their eyespots are covered.

     As is seen in all echinoderms, starfish anatomy consists of possessing a hydraulic water vascular system that helps them in locomotion. The water vascular system has many projections known as tube feet that emerge through openings inside the endoskeleton. As there are more than 1800 known species of starfish, hence it is not possible to simply state what is starfish anatomy. However, this article has described the basic internal and external anatomy of starfish while doing a starfish dissection that is common to most species of starfish.

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