(I) Alimentary canal:
Alimentary canal is complete and lies along the oral-aboral axis and is quite short due to flattening of the body. It has following parts.
Image Source: images.slideplayer.com
ADVERTISEMENTS:
Mouth:
It is a pentagonal aperture, surrounded by peristome and guarded by five groups of mouth papillae or oral spines.
Muscles of the body wall surrounding mouth, act as a sphincter for closing and opening the aperture.
Oesophagus:
ADVERTISEMENTS:
Mouth opens into a short and wide oesophagus. It extends vertically to open into the stomach.
Stomach:
It occupies most of the space of the central disc. It is differentiated into a larger oral part or cardiac stomach and a similar aboral part or pyloric stomach seperated by a constriction.
(a) Cardiac stomach:
ADVERTISEMENTS:
It is thin walled muscular and highly folded part and everted out from the mouth during feeding.
It comes back to its position with the help of retractor muscles. They are homologous to mesentries in vertebrates. Cardiac stomach is glandular and secretes mucus.
(b) Pyloric stomach:
Behind the cardiac stomach towards aboral surface the pyloric stomach is situated. It receives pyloric duct from each pyloric caeca of each arm at five angles.
(c) Pyloric caeca or digestive gland:
A pair of caeca extends upto the tip of each arm and is suspended from the aboral body wall by a pair of longitudinal mesenteries.
Each caeca consists of a hollow longitudinal axis and the two series of lateral hollow branches; one on either side arises from it. Both the caeca communicate pyloric stomach through a common pyloric duct.
The epithelial wall of the caecum contains four types of flagellated columnar cells
(i) Secretary or granular cells, which secrete proteolytic, annylolytic and lipolytic enzymes
(ii) Mucus cells, secrete mucus
(i) storage cells, which store and reserve food in the form of lipids, glycogen and protein-polysaccharides complex and
(iv) Current producing cells which bear longer flagella.
The current producting cells maintain a steady flow of enzymes towards the stomach along the aboral side and of digested food into pyloric caeca along the oral side.
Intestine:
It is a short five sided tube extending vertically to aboral surface which opens out by an anus. It gives off two or three branches, the intestinal caeca at inter-radial position.
These caecae secrete a brownish fluid probably excretory in nature. Remaining part of the intestine after caecae is sometimes called the rectum.
Anus- It is a small rounded aperture situated somewhat eccentrically on aboral surface of the central disc.
(II) Food and Feeding mechanism:
Sea star is a voracious carnivore. It preys upon sedentary marine animals like, clams, oysters, mussels, snails, crabs, branacles worms and even sea urchins and other smaller starfishes. At times it also feeds upon small fishes and injured and dead animals.
Prey is captured with the help of arms and tube feet. The mechanism of preying is very interesting and worth to seeing.
(III) Digestion, absorption and egestion:
During feeding, digestive enzymes secreted by stomach and pyloric caecae are poured over the prey. Digestion thus takes place outside the body.
Digestion of semi-digested materials if any, is completed in stomach and pyloric caecae. In pyloric caecae, it is believed that some intracellular digestion takes place.
Absorption of digested food occurs mainly by pyloric caecae and distributed to various parts of the body by coelomic fluid. Excess food is stored in the storage cells of the pyloric caecae.
Egestion takes place through the anus.