The lateral walls of pharynx are perforated by 150-200 pairs of closely and obliquely set narrow and vertical opening called branchial apertures gill-slits or gill-clefts. The gill-slits bear no gill and their number increages with the age of the animal on the posterior .The portion of pharyngeal wall between two gill slits is called a branchial lamella or gill bar. Gill bar consists of a ciliated epithelial covering both ectodermal, enclosing a mesodermal core containing fibrous connective skeletal rod.
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