1. Explain the challenges and advantages of having internal gills over external

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.....---->>>>External Gills: These are rarely seen in fishes but found in larval forms of some. One pair is seen in larva of polypterus and four pairs in dipnoi larvae. They disappear in adult stages. They are present in larval forms of most amphibians. The integument covering visceral arches form tufts of filaments which are seen as external gills. During embryo development five pairs of pharyngeal pouches arise of which second, third and fourth perforate and open to exterior. The arterial loops coming from the aortic arches ramify in the external gills. These external gills are covered with cilia and are in direct contact with water. The exchange of gases occur through their surface epithelium. In all tailless amphibians and in some tailed amphibians these are lost during metamorphosis........................ .....---->>>> Internal Gills: During the tadpole development an operculum arises and covers the gill clefts and gills. Thus the gills are now enclosed in an opercular chamber. These gills degenerate to give rise to a new set of gills called internal gills from the same visceral arches. The gills and operculum are not similar to that in fishes. The pharyngeal pouches also called gill clefts are compressed anterior-posteriorly and communicate with the cavity of pharynx through a large internal branchial aperture. They open outside through an exterior branchial aperture also called gill slit. The endodermal mucous membrane of gill pouch is raised into a series of horizontal folds to form lamelliform branchial lamellae. The branchial lamellae have a rich blood supply covered with thin membrane. Two lamellae are present in each gill pouch and each are called hemibranch. The inter branchial septa separates gill pouches. This inter branchial septa is supported externally by a gill ray which arises from visceral arch. The visceral arch also give rise to gill-raker to support internal branchial aperture.

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