Describe the respiratory system of Scolidon
Answers
There are five pairs of gill- pouches bearing gills, arranged in a series behind the hyoid arch in the lateral walls of the pharynx. Each gill-pouch is compressed antero-posteriorly and communicates with the cavity of the pharynx through a large internal branchial aperture and with the exterior through a narrow external branchial aperture (commonly called gill- slit). The endodermal mucous membrane of gill-pouches or interbranchial septa is raised into a series of horizontal folds to form lamelliforms branchial lamellae or gill-filaments.
Pharynx Exposed to Show Gill-Pouches
The branchial lamellae have a rich blood supply, and they have a very thin covering membrane through which blood is exposed to sea water for an exchange of gases. Each gill-pouch has two sets of gill-lamellae, one on its anterior wall and the other on the posterior.
Each set of lamellae is a half gill or hemibranch, so that gill-pouch has two hemibranches. The gill-pouches are separated from each other by fibro-muscular partition called the inter-branchial septa (they should more correctly be called intra-branchial septa because each lies between two successive gill-pouches).
The interbranchial septa extend well beyond the branchial lamellae, then each bends posteriorly to form a flap which protect the lamellae as well as external gill-slit. The inner part of each interbranchial septum has a supporting cartilaginous visceral arch with slender gill-rays. Visceral arches also give out rigid comb-like gill-rakers which project inwards to protect the internal branchial apertures from entering the food.
Each visceral arch supports the posterior branchial lamellae or hemibranch of posterior gill-pouch and the anterior branchial lamellae of the anterior gill-cleft. These two hemibranchs or demibranchs with their interbranchial septum and the visceral arch constitute a complete gill or holobranch.
The posterior hemibranch of a septum has longer lamellae than the anterior one. Thus, a gill-pouch has two demibranchs belonging to two adjacent gills. In Dogfish (Scoliodon), the hyoid arch bears only a hemibranch on its posterior surface, the first four branchial arches has holobranchs and the fifth branchial arch is without any gill (abranch). Thus, it has nine hemibranchs.
Respiratory System in T.S. of Body
Between the mandibular and hyoid arches or the first internal gill-slit on either lateral side of pharynx is a spiracle. In most elasmobranchs the spiracle bears minute branchial lamellae forming a false gill or psendobranch and opens to the exterior by an external branchial aperture. It is supplied with arterial blood and plays no part in respiration. But in Dogfish (Scoliodon) the spiracles are vestigial pits in the pharynx with no lamellae and external branchial apertures.
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