scolidon pharynx exposed to show gill-pouches
Answers
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.
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