2. Habits and habitat
• Marine
• Carnivorous
• Feeds on crabs, lobsters,
worms and fishes
• Sexes are separate
• Internal fertilization
• Direct development
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3. Body
• Elongated, fusiform (spindle shaped) body.
• Fully grown fish measures about 60 cm.
• Body divided into head, trunk and tail.
• Fins: fins are flap like outgrowth of the body wall, internally
supported by cartilaginous rods.
• Fins are both paired and unpaired.
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4. Skin
• The skin has a
coarse texture, and
gives a feel of sand
paper.
• Placoid scales
remain embedded
into the skin
4
6. Digestive system
• The digestive system includes the alimentary
canal or gut and the glands that open into it
• Digestion is the process of simplification
of the complex food material, which can be
absorbed and assimilated by the body
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7. Alimentary canal
• It is complete and
longer than the body
and includes buccal
cavity, pharynx,
oesophagus, stomach
and intestine.
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8. Mouth
• It is a semi-oval slit
bounded by upper
and lower lips and
is present at the
ventral side.
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9. Buccal cavity
• 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 merely a thick, flat, non-muscular, non-glandular and
non-protrusible fold of mucous membrane supported
internally by the flat cartilage
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10. Pharynx
• 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
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11. 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
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12. 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
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13. 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
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14. 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
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15. 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
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16. 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
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17. Food and feeding
• It is carnivorous, feeding on crustacean, annelids
and small fishes. It is voracious feeder and can
smell the food from long distances.
• Due to ventral position of mouth it has to turn to
one side to capture prey. The lower jaw is opened
by coraco mandibular muscles. The escape of
struggling prey is prevented by backward directed
teeth.
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18. 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
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19. References
• Kotpal, R. L. (2010), Modern text book of
Zoology: Vertebrates, Rastogi Publications,
New Delhi, India
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