In Scoliodon respiration takes place through 5 pairs of gill slits or gill pouches. They are present in a series on the wall of pharynx on either lateral side, behind the hyoid arch.
Each gill pouch opens into the pharynx by a large branchial aperture and to outside through an external branchial aperture or gill slit.
Two adjacent gill pouches are completely separated by a vertical fibro-muscular partition, the interbranchial or gill septum.
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The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays. The septum is covered by epithelium and contains blood vessels, nerves etc.
The mucus membrane of a septum is raised into numerous horizontal leaf like folds called gill lamellae or gill filaments.
These constitute the gill proper and are richly supplied with blood capillaries. Each septum bears two sets of gill-lamellae, one on its anterior face and the other on its posterior face. Each set makes a half gill called demibranch.
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A gill pouch thus contains two demibranchs belonging to two different adjacent gills. In Scoliodon, the hyoid arch bears only a demibranch on its posterior face.
The first four branchial arches bear holobranchs while the fifth one is devoid of gills and is called abranch.
In front of hyoid arch or the first internal gill slit is an oval pit of spiracle. It has no gill lameliae or external opening and is regarded as vestigeal gill pouch.
However, in some other elasmabranchs it bears minute gill lamellae and opening, which is called pseudobranch.
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Mechanism of respiration
1. Inspiration:
During inspiration the bucco-pharyngeal cavity expands by the action of hypo-branchial muscles.
As the external gill slits are tightly closed water enters the enlarged cavity through the open mouth.
Breathing movements occur in two steps:
2. Expiration:
Next when the mouth is closed by the action of adductor muscles the volume of bucco-pharyngeal cavity gets reduced due to the contraction of constrictor and interbranchial muscles.
As a result water is forced to enter the gill pouches, over the gill lamella. Water finally goes out through the open gill slits.
Physiology of Respiration :
Each gill lamella has an extensive system of sinusoids which receive venous blood from an afferent branchial artery and pass it on to an efferent or epibranchial artery.
During the passage of blood through this network it becomes oxygenated by exchanging O2 and CO2 between the water and blood. This process is called external respiration.
As this oxygenated blood circulates through body its O2 is used by tissues to oxidise food stuffs and CO2 is released into blood which is formed during oxidization of food stuffs. This process is called internal respiration.