Development and structure of pharynx. Fate of 1st pharyngeal pouch. 1st visceral arch becomes jaws of gnathostomes. General structure and function. Each gill bar may be extended by gill septa.
Most bars have gill rakers on their pharyngeal margins.
Typical gills-two rows of gill filaments . Capillary beds in lamellae between the loops. Small barrier between blood and water. Fine mesh slows water down. Gills also important in excretion and osmoregulation .