Starfish shows an alimentary canal extending from oral to aboral side. It shows the following parts.
1. Mouth: It is present on the oral surface. It shows sphincter muscles. It is capable of great expansion and redution.
2. Oesophagus: Mouth leads into short oesophagus. This leads into stomach.
3. Stomach: It shows cardiac and pyloric parts. The first part of the cardiac stomach is big and 5 lobed. It can come out through mouth. This is brought forward by 5 pairs of retractor muscles.
The pyloric stomach is small and star shaped. Each angle of the stomach is drawn into the corresponding arm. This is branched. These branches are called pyloric caecae or hepatic Caecae, or gastric glands. They work like pancreas. They secrete digestive juices. They store reserve food.
4. Intestine: Pyloric stomach opens into 5 sided broad intestine. From intestine 2 to 3 rectal caecae are given off.
The last part of the intestine is called rectum, which opens out through anus.
Star fish is a carnivorous animal. It feeds on a number of sedentary marine animals. Ex: Oysters, Crustaceans etc.
Whenever it comes in contact with an oyster, it archs over it. With the help of tube feet, it separates the two valves of the oyster. Through this gap it pushes its cardiac stomach into the mantle cavity of the oyster.
Digestion is both extra and intracellular. The digestive juices are poured on the soft parts of the prey. The digestion is partly completed outside the body. Then star fish withdraws its stomach along with the digested food. The remaining digestion is completed" in the stomach and. pyloric caecae. The digestion is completed in the stomach and pyloric caecae. The digested food is absorbed through the walls of the digestive tract into the coelome and supplied to all parts. Most of the undigested food is sent out through mouth. In star fish very small amount of undigested food will be sent out through anus, hence the intestine and rectum are very small.