Describe the structure of gill and mode of respiration in scolidon
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Tejass
The respiratory organs are the gills which are borne by the gill-pouches. The structure of gill-pouches differs in different dogfishes. Fig. 6.8B shows the organisation of gill-pouch of Brachaelurus, a related genus of Scoliodon. There are five pairs of gill-pouches, each of which communicates with the pharyngeal cavity by a large internal branchial aperture and opens to the outside by exterior gill-slits The mucous membrane lining of the gill- pouches gives a series of horizontal branchial lamellae. The branchial lamellae are highly vascularized structures. Each gill-pouch has an anterior set of branchial lamellae and a posterior set of branchial lamellae. The gill- pouches are separated by inter-branchial septum which projects beyond the branchial lamellae (Fig. 6.8C).
The pharyngeal end of each inter-branchial septum is supported by a visceral arch. Each arch supports the anterior set of lamellae of one gill-pouch behind and the posterior set of lamellae of the next gill- pouch. The first gill-pouch lies between the hyoid and the first branchial arches and the last one is present between the fourth and fifth branchial arches.
There are two types of gills:
(i) Holobranch or complete gill when a branchial arch bears two sets of gill lamellae and
(ii) Demi branch or hemi branch or half gill when single set of gill lamellae is present. The hyoid arch supports only a demi branch and the first four branchial arches support holobranchs. The last branchial arch is gill-less.
Mechanism of respiration:
During respiration the floor of the buccal cavity is lowered and the mouth is opened. Then the water rushes in to fill the greatly expanded buccal cavity. The mouth is now closed and the pharynx contracts.
The water then enters the gill-pouches and goes out after gaseous exchange through gill-slits. The spiracles are occasionally used as accessory pathways for the entry of water for respiration instead of the mouth when it is otherwise occupied.
Answers & Comments
The pharyngeal end of each inter-branchial septum is supported by a visceral arch. Each arch supports the anterior set of lamellae of one gill-pouch behind and the posterior set of lamellae of the next gill- pouch. The first gill-pouch lies between the hyoid and the first branchial arches and the last one is present between the fourth and fifth branchial arches.
There are two types of gills:
(i) Holobranch or complete gill when a branchial arch bears two sets of gill lamellae and
(ii) Demi branch or hemi branch or half gill when single set of gill lamellae is present. The hyoid arch supports only a demi branch and the first four branchial arches support holobranchs. The last branchial arch is gill-less.
Mechanism of respiration:
During respiration the floor of the buccal cavity is lowered and the mouth is opened. Then the water rushes in to fill the greatly expanded buccal cavity. The mouth is now closed and the pharynx contracts.
The water then enters the gill-pouches and goes out after gaseous exchange through gill-slits. The spiracles are occasionally used as accessory pathways for the entry of water for respiration instead of the mouth when it is otherwise occupied.