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Digestive System<br /> Starfish have two stomachs- the cardiac stomach and the pyloric stomach. The cardiac is a sack-like stomach found in the center of the body. It can be pushed out of the starfish’s body and used to engulf food. Some starfish force open mollusks (like clams) by injecting their stomachs into the shells. With the stomach inside, the starfish digests the mollusk in the shell. The pyloric stomach are connected by two pyloric caeca- hollow tubes that absorbs nutrient from their food. Starfish eat many things, like barnacles, snails, sea urchins, clams, and mussels.<br />
Excretory System<br />Starfish have no excretory organs, but waste is excreted through the tube feet and papullae*.<br /> tube feet<br />*tiny structures that dot the surface of starfishes.<br />
Nervous System<br />Starfish have complex nervous systems, but don’t have a brain. The nervous system is a radical nerve that’s in each arm, and a circular nerve that connect all the radical nerves around the mouth. However, all the “thinking” is done in the radical nerves (instead of the circular one). The nerves coordinate the starfishes’ balance and directions. They are very sensitive to touch, light, temperature, and orientation.<br />
Circulatory System<br /> Circulation is found in three places- the perivisceralcoelom (extra space inside the body), the water vascular system (tube feet), and the hemal system. The tube feet retract and extend to keep things moving. Cirulation in the perivisceralcoelom is from ciliary beating.<br />The hermal works by forming rings around the mouth (the oral hemal ring), upper surface (the aboralhemal ring), and the digestive system (the gastric hemal ring). The rings are connected by the axial sinus. The “heart” is connected to the axial sinus, and seems very incapable because it lacks a one way valve system. The hemal system is for distributing nutrients from the digestion tract.<br />
Reproductive System<br /> Starfish are able to reproduce asexually and sexually. Most species are either male or female, but there are some that are both- hermaphrodites. Fertilization occurs externally, by releasing their gametes into the environment. Fertilized eggs grow into bipinnarialarve, and then brachiolaria larvae, living as zooplankton, and eventually sink to the bottom and grow into adults. However, some starfish produce asexually when their arm is detached, causing the arm to develop into a starfish. But it can only evolve into a new starfish if some of the central disk came along with it.<br />bipinnarialarve<br />brachiolaria larvae<br />
Respiratory System<br /> Opposite to the excretory system, oxygen enters by the tube feet or papullae. Oxygen is then evenly distributed by the main body cavity. The hemal system is also involved with the respiratory system.<br />
Muscular/Skeletal System<br /> Starfish have mesodermal* endoskeleton**, with small calcareous ossicles(bony plates). Their bones are very flexible, allowing them to move around freely. Each starfish has hundreds of tube feet, and they’re filled with ocean water. The water vascular system moves the water into the tube feet, allowing them to move an arm (by expanding it). Muscles within the feet retracts them.<br />endoskeleton<br />*middle layer that develops muscle, bone, cartilage, blood, and connective tissues<br />**internal supporting skeleton <br />
Body Plan<br /> Starfish are typically seen as having five arms. But there can be more or less, depending on the species. However, some may have more or less even within the same species. They have hundreds of tiny feet in one arm- tube feet. The mouth is under the starfish, while the anus is on the top. There is a structure called madreporite, a small white spot that filters water and supplies water to the water vascular system.<br />
Habitat<br /> Starfish can live anywhere in the ocean, but most are found in rocky places (close to the waters). They can swim to the bottom of the sea, or float on the ocean surface. Some live on coral reefs, while others live in small caves made by other sea animals. <br />