In scoliodon, the respiration is aquatic, i.e., respiration in water and it breathes by means of gills borne in a series of gill pouches on either side of the pharynx. Water enters the mouth and after passing through the buccal cavity, pharynx, gill-pouches and goes out through the gill slits after bathing the gills.
There are five pairs of lathral gill-pouches situated in the latheral walls of the pharynx and are arranged in a series on their side. Each gill-pouch is compressed anteroposteriorly and communicates with the cavity of the pharynx through a large internal branchial aperture and with the exterior through a narrow external branchial aperture (commomly called gill slit).
The endodermal mucous membrane of gill-pouches is raised in to a series of horizontal folds to form lamelliform branchial lamellae. The bbranchial lamellae have a rich blood supply, and they have a very thin covering membrance through which blood is exposed to seawater for an exchange of gases.
Each gill-pouch has two sets of gill-limellae is a half gill or hemi branch, so that gill-pouch has two by muscular interbranchial septa of connective tissue (they should more correctly be called interabranchial septa because each lies between two successive gill-pouches). The interbranchial septa extend well beyond the branchial lamellae, and then each bends posteriorly to protect the lamellae.
The inner part of each interbrachial septum has a supporting visceral arch from which cartilaginous gill rays arise in a single row and project into the interbranchial septum for further support. Visceral arches also give out rigid comb-like gill rackers, which project the internal branchial apertures from food.
Each visceral arch supports the posterior branchial lamellae or hemibranch of one gill-pouch and the anterior branchial lamellae, or hemibranch of the gill cleft behind it. These two hemibranches with their interbranchial septum and the visceral arch constitute a complete gill or holobranch. The posterior hemibranch of each holobranch is larger than the anterior one.
Between the mandibular hyoid arches of each side is a spiracle. In most plasmobranches the spiracle bears branchial lamellae and opens to the exterior by an external branchial aperture. It is supplied with arterial blood and plays no part in respiration. But in scoliodon the spiracles are vestigial pits in the pharynx with no lamellae or external branchial aperture. The hyoid arch bears only a hemibranch posteriorly. The first four branchial arches bear a holobranch each, the fifth braanchial arch has no branchial lamellae, and thus scoliodon has nine hemibranches on each side.
Mechanism of Respiration:
The floor of the buccopharyngeal cavity is despressed by hypobranchial (hypoglossal) muscles and the mouth is opened at the same time the viscera arches expand the wall of the pharynx, so that see-water containing dissolved oxygen rushes in through the mouth.
Entry of the water into external brachial apertures is prevented by an anterior fold is then raised and the mouth is closed, and contractions of the wall of the pharynx force the water into internal branchial apertures, the oesophagus being closed, and then into gill –clefts, where it washes the branchial lamellae and goes out of the external branchial apertures.
In the branchial lamellae, the blood flows from the tip towards the base that is in a direction opposite to that of the water current, so that the blood just before leaving the jamellae meets the highest concerntration of oxygen and the lowest of carbon dioxide takes place between the blood and seawater. The respiratory movements are caused by pharyngeal muscles, which are innervated by V, VII, and IX and X cranial nerves and the phyeglossal spinal nerve.
Physiology of Respiration:
Fresh seawater entering the gill pouches with the respiratory current contained within the capillaries of the gill lamellae merely by the thin and permeable membranous walls of the capillaries, the oxygen of the water passes by endosmosis through the thin capillaries walls into the blood passes into the water by a process of exomosis.
The oxygen is converted by the blood to all the parts of the body, while carbon dioxide brought to the gill in the venous blood is eliminated by the water of the outgoing respiratory current. As the blood makes a complete circuit in the capillaries of the gills in a very short time, it is evident that exchange of gases also takes place very quickly.