In Dogfish (Scoliodon), the respiration is aquatic, since the animal resides in water. It breathes by means of gills borne in a series of gill-pouches on either lateral side of the pharynx. Water enters the mouth and after passing through the buccal cavity, pharynx, gill- pouches bearing gill-lamallae, goes out through the external gill-slits after bathing the gills.
Respiratory Organs:
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.
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.
ADVERTISEMENTS:
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 fi