Respiratory organs of Cyclostomes

• Agnathans have 6-15 pairs of gill pouches, which are lateral extensions of pharynx and contain gill lamellae within.
• Cyclostomes are called marsipobranchs, which means “pouched gills”, since the gill lamellae are housed in gill pouches.
• The hagfish, Myxine has only 6 pairs of gill pouches whose ducts join together and open to the exterior by a single pair of openings, while Bdellostoma carries 6-15 pairs of gill pouches that vary in different species and open to the outside independently.
• In Myxine behind the gill pouches there is a single pharyngo-cutaneous duct on the left side, which is a modified gill pouch which drains excess water that fails to enter the gill pouches.
• The lamprey, Petromyzon, has 8 embryonic and 7 adult paired gill pouches that open to the exterior by independent openings.