The pharyngeal wall of branchiostoma is richly vascular and the warer current entering the pharyngeal cavity brings fresh oxygen. The blood flows so close to the surface that some exchange between carbon-di-oxideof blood and oxygen of water can easily occur. There is no capillary network in gill bars. The blood lacks a respiratory pigment. Many workers questions about the respiratiory role of pharynx. It is probable that most gaseous exchange occurs through superficial areas such as fins, metapleural folds and atrial wall containing lymph spaces.
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