Scoliodon – Dogfish, Classification, and Characteristics
Systemic Position of Scoliodon
Habit and Habitat:
The Scoliodons are widely distributed in India, Pacific West India, and the eastern coast of South America and Atlantic Oceans.
The common Indian genus is S sorrakowah “Black Shark” – Dog Fishes. These are marine and found in all open seas.
Body of Scoliodon:
- The body of Scoliodon is elongated, spindle-shaped, body tapered at ends, making it a very fast swimmer.
- The trunk and tail are laterally compressed, and the head region is dorsoventrally compressed.
- The entire body is covered by an exoskeleton of placoid scales.
- The mouth is located on the ventral side, bound on both sides by jaws.
- Two crescentic apertures, the nares or nostrils are present ventrolaterally and anterior to mouth. These are exclusively olfactory, no role in respiration.
- It consists of two rows of homodont or polyphyodont teeth, these are homologous to the placoid scales.
Fins of Scoliodon:
These are thin, flat-out growth of skin with muscles and are supported by cartilaginous rods or rays.
- It consists 1st dorsal fins, Second dorsal fin, caudal fin, anal fin, and paired fins (anterior pair pectoral fins and posterior-lateral pelvic fins).
- The pelvic fins are simple but in males, the inner margin of the pelvic fins bears a pair of rod-shaped copulatory organs called clasper or myxipterygium. The claspers are the intermittent organs that introduce sperm into the female genital tract.
- The fins propel the body in forwarding direction during swimming
External Gill slits: Anterior to each pectoral fin, on either side of the body vertically elongated external gill slits or branchial clefts are present in a series of 1 to 5. They are the main respiratory organs.
Cloacal Aperture: Cloacal aperture is found on the tail region, between two Pelvic fins, it leads a small chamber called the cloaca, which is the common exit for the digestive and urinogenital system.
Caudal Pits: These are found at the base of the caudal fin, the tail bears two shallow depressions, one dorsal and one ventral, known as caudal pits. Caudal pits are characteristic features of the genus scoliodon.
The digestive system consists of the alimentary canal and associated glands
It is complete and is divided into the buccal cavity, pharynx, oesophagus, stomach, intestine and rectum. The stomodeal portion is lined with the ectoderm. The mesodaeum is lined with endoderm. The proctodaeum is also lined with the ectoderm.
Mouth: – It is a semi-oval slit bounded by upper and lower lips and is present at the ventral side.
Buccal Cavity: It is a wide cavity containing sharp-pointed backwardly directed homodont and polyphyodont 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.
Pharynx: A large wide chamber opens through five vertical slits called internal gills slits which open into gills. The mucous membrane contains numerous denticles to prevent the escape of prey.
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 the stomach. It opens into the stomach, through the oesophageal valve.
Stomach: it is “J” shaped tube divided into two parts:
- Cardiac Stomach: It is a long, broad muscular part containing longitudinal folds. At the end there is a cardiac valve
- Blind Sac: At the end of cardiac stomach starts there us a small, outgrowth called blind sac of unknown function
Intestine: divided into duodenum and ileum.
- Liver: Large yellowish two-lobed gland present in-between stomach and is suspended by falciform ligaments, gastrohepatic omentum, and hepatoduodenal ligament.
- The gall – bladder stores bile and drains by a cystic duct which fuses with hepatic duct to form common bile duct, which opens into the duodenum.
- 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 the ventral lobe is closed to the pyloric stomach. The pancreatic duct opens into the ventral wall of the duodenum.
In Scoliodon the respiration takes place through 5 pairs of gill slits or gill pouches.
- The gills are present in a series on the wall of the pharynx on either lateral side, behind the hyoid arch
- Each gill pouch opens into the pharynx by a large branchial aperture and to the outside through an external branchial aperture or gill slit
- Two adjacent gill pouches are completely separated by a vertical fibro-muscular partition, the inter – branchial or gills septum.
- The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays.
- A gill pouch contains two demibranchs belonging to two different adjacent gills. In Scoliodon, the hyoid arch bears only a demibranch on its posterior face.
The circulatory system consists
- Colourless blood.
- Heart: comprising receiving part: sinus venousus and a dorsally placed auricle, forwarding parts consisting of the ventricle and a conous arteriosus. The heart is situated on the ventral side of the body, between the two series of gill pouches.
- Arteries: Afferent branchial arteries, arise from the ventral aorta bring deoxygenated blood to gills. Efferent branchial arteries- originate from gills and transport oxygenated blood to different body parts.
The nervous system in scoliodon:
The brain is developed, divided in three parts – Forebrain, midbrain, and Hindbrain.
Cranial nerves in Scoliodon: There are ten pairs of cranial nerves are found in all fishes, an extra pair of pre olfactory nerves are present in Scoliodon.
Ampulla of Lorenzini: the enumerable pores on the dorsal and ventral sides of the head leads into a long tube that terminates into radially arranged ampullary sacs. In Ampulla two types of cells are found – glandular cells and Sensory cells. The sense organs are pressure and thermoreceptors.
Development in Scoliodon:
These are viviparous and give birth to living young.
- Fertilization is internal.
- Telolecithal eggs are found in them.
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