Brief note on the digestive system of Dog fish or Scoliodon

Introduction:

Digestion is the process of simplification of the complex food material, which can be absorbed and assimilated by the body.

Digestive system:

Digestive system consists of alimentary canal and associated glands.

1. Alimentary canal:

It is complete and is divided into buccal cavity, pharynx, oesophagus, stomach, intestine and rectum. The stomodeal portion is lined with ectoderm. The mesodaeum is lined with endoderm. The proctodaeum is also lined with ectoderm.

2. Mouth:

It is a semioval slit bounded by upper and lower lips and is present at the ventral side.

(i) Buccal Cavity:

It is a wide cavity containing sharp pointed backwardly directed homodont and polyphyodent type of teeth. These are mainly used merely to prevent the escape of prey. The mucous membrane is raised to form the tongue which is glandular and muscular.

(ii) Pharynx:

It is a large wide chamber which opens through five vertical slits called internal gillslits which open into gills. The mucous membrane contains numerous denticles to prevent the escape of prey.

(iii) Oesophagus :

The pharynx opens into a short narrow thick, muscular oesophagus containing rugae. It remains closed, except during swallowing of food, to check the entry of water into stomach. It opens into stomach, through oesophageal valve.

(iv) Stomach:

It is ‘J’ shaped bent tube which is divided into two parts.

(a) Cardiac Stomach: It is a long, broad muscular part containing longitudinal folds. At the end there is a cardiac valve.

(b) Blind Sac:

At the end of cardiacs stomach starts there us a small, outgrowth called blindsac of unknown function.

(c) Pyloric Stomach:

The distal limb of stomach starts from cardiac valve and is a short narrow tube. IT is lined with smooth, mucous membrane with few longitudinal folds. It ends at the pyloric valve.

(d) Bursa entiana :

At the distal end of pyloric stomach a small globular, muscular sac is present containing continuation of longitudinal folds of pyloric stomach is called Bursaentiana.

The stomach is suspended by mesogaster in the coelom.

(v) Intestine:

It is a straight wide tube which is divided into duodenum and ileum.

(a) Duodenum:

It is the narrow, anterior part. It receives common bile duct or ductus choledochus dorsally and pancreatic duct ventrally.

(b) Ileum :

The rest part of intestine is called as ileum. The mucous membrane is produced into scroll valve. One edge if the is valve is attached to the intestinal mucosa. But the other end roles up spirally in anticlockwise manner upto two and half turn. The scroll valve increases the inner absorptive area. The passage of food slows down in scroll valve to ensure proper absorption.

The intestine is not connected with mesentery and is opened into the rectum.

Rectum:

It is a short narrow straight tube suspended by mesorectum. In between ileum and rectum there is rectal valve containing circular muscles. It receives rectal gland which is similar to medulla of kidney and excretes excess salts. Rectum leads to cloaca.

Cloaca:

It is a short terminal chamber, which receives ducts of urinogenital system. It has a pair of abdominal pores.

Cloacal aperture :

It is a small longitudinal aperture present between two pelvic fins.

2. Digestive glands:

Liver is a large yellowish two lobed gland present in between stomach and is suspended by falciform ligaments,gastrohepatic omentum and hepatoduodenal ligament. The V-shaped gall-bladder stores bile and drains by cystic duct which fuses with hepatic duct to form common bile duct. It opens into deudenum.

3. Pancreas:

It is an elongated, whitish gland present between two limbs of the stomach. It is divided into two lobes. The dorsal lobe is present parallel to the posterior part of the cardiac stomach and ventral lobe, closed to the pyloric stomach. The pancreatic duct opens into the ventral wall of the duodenum.

4. Gastric glands:

These are present in the stomach.

5. Intestinal gland:

Numerous microscopic glands of intestine secrete succusentericus.

6. Mucous glands:

Mucous glands are found in the inside alimentary canal which helps to dissolve the food and makes the passage slippery.

Food: It is carnivorous, feeding on crustecian, annelids and small fishes. It is a voracious feeder and can smell the food from a long distance.

Feeding: Due to ventral position of mouth it has to turn to one side to capture the prey. The lower jaw is opened by coraco mandibular muscles. The escape of struggling prey is prevented by backwardly directed teeth. Without mastication it is swallowed.

Digestion:

1. Stomach:

The gastric juice contains pepsin along with HCl to convert proteins into peptone and proteoses.

2. Duodenum:

The food in duodenum is acted upon by bile and pancreatic juice trypsin and amylopsin digest protein and starch but lipase with the help of bile digests fat.

Absorption:

The spiral valve extends to the area of absorption and slows down the passage of food or absorption.

Assimilation:

Due to large supply of food, little storage is seen. Glycogens and oils are temporarily stored in the liver cells.

Egestion – The waste material are thrown out through cloacal aperture as faeces.