The digestive system includes the alimentary canal or gut and the glands that open into it.
Alimentary canal:
It is complete and longer than the body and includes buccal cavity, pharynx, oesophagus, stomach and intestine.
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(1) Mouth:
It is a wide crescentic opening on the ventral side of the head. It is bounded by folds of integuments sometimes called upper and lower lips.
(2) Buccal cavity:
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Mouth leads into a spacious dorso-ventrally flattened mouth cavity bordered by the jaws. Teeth are homodont and are embedded in the skin, sharply pointed and directed backwards.
They are polyphyodont (replaced several times during the life) and arranged in many rows. Teeth help in grasping the prey.
On the floor of the buccal cavity lies the so called tongue. It is meraly a thick, flat, non-muscular, non-glandular and non-protrusible fold of mucous membrane supported internally by the flat basihyal cartilage.
(3) Pharynx:
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Posteriorly buccal cavity opens into pharynx which is lined by endoderm. Each lateral side of pharynx contains an oval pit of spiracle and five separate gill slits.
(4) Esophagus:
Pharyngeal cavity narrows down posteriorly into a short but wide tube, the oesophagus, with thick muscular wall. Its mucus lining is thrown into longitudinal folds.
(5) Stomach:
Stomach is a U-shaped large cavity in which oesophagus opens. Its proximal limb, the cardiac stomach is longer, wider and distensible.
Its distal limb is shorter and narrower and is called the pyloric stomach. The opening of oesophagus into cardiac stomach is guarded by an oesophgeal valve.
The mucus lining of cardiac stomach also forms longitudinal folds like those of oesophagus. At the junction of cardiac and pyloric stomach is present a small blind outgrowth, the blind sac as well as a sphincter valve.
The lining of pyloric stomach is mostly smooth. At the end of pyloric stomach is present a strong circular muscle band called pyloric valve guarding its opening into a small but thick-walled muscular chamber, the bursa entiana.
(6) Intestine:
Bursa entiana is followed by intestine. It is a straight wide tube. Its narrow anterior part receives the bile and secretions of pancreatic ducts.
In scoliodon, the mucus lining of intestine becomes folded anticlockwise into a longitudinal spiral or scroll of about two and a half turns.
This is called the scroll valve or spiral valve. It serves to delay the passage of food and offers increased surface for absorption like the typhlosole of earthworm.
The last part of intestine is called rectum. It is a short and narrow tube opening behind through anus into the ventral cloaca.
A small finger-like cloecal or rectal gland of unknown function opens dorsally into the rectum.
(7) Glands of alimentary canal
Liver:
It is a massive yellowish bilobed gland. The two lobes extend backward freely into abdominal cavity, but they are united anteriorly and are attached to septum transversum by a ligament.
A V-shaped thin walled gall bladder, in which bile is collected, lies embedded in the right lobe of liver.
A narrow bile duct, about 3 cm long, leaves the gall bladder and opens into the anterior end of the intestine.
Bile duct also receives branches from the lobes of liver. It secretes bile, stores glycogen and fat and destroys worn out erythrocytes of blood.
(8) Pancreas:
It is a compact whitish or pale bilobed gland consisting of a longer dorsal lobe running parallel to the posterior part of cardiac stomach and a smaller ventral lobe closely applied to the pyloric stomach. The small pancreatic duct traverses the entire length of the gland to open into the intestine just opposite the opening of the bile duct.
(9) Caecal or rectal gland:
It is a small finger like body attached by its duct to the dorsal side of rectum into which it opens. It is highly vascular and composed of lymphoid tissue. Its function is unknown.
(10) Spleen:
It is a large gland closely attached like a fringe to the cardiac and pyloric stomach. But it has no physiological relation with alimentary canal. It is a lymphoid organ which produces lymphocytes.
Food and physiology of digestion:
Scoliodon is a predaceous carnivore feeding mainly on other fishes. Its diet may also include crabs, lobsters and worms.
Food as a whole is swallowed; no digestion takes place in buccal cavity. Main digestion occurs in the stomach by the action of Pepsin and HCI of gastric juice.
Bile and pancreatic juice containing trypsinogen, amylopsin and lipase act upon the semi-digested food in the intestine.
Scroll valve in intestine serves to retard the speed of passage of food to extend the time of digestion and increase the surface of absorption.