RESPIRATORY ORGANS
Open the anterior gill pouch of the left side by dorsal and ventral cuts extending from the angles of the cleft, but cutting only as far as is necessary to see the structures within the pouch. Upon the medial side the gill pouch opens into the pharynx by a dorso-ventral slit, guarded by projecting cartilagineous gill rakers, which prevent particles of food from passing into the gill pouch with the respiratory current. On both the anterior and posterior wall of the pouch is a demibranch. If the specimen is injected a large blood vessel can be seen through the skin in the inner border of the demibranch, and small vessels passing from this into the leaflets, where the interchange of gases between the water and blood takes place.
Open similarly each pouch of the same side, observing the number of the demibranchs and their relation to the pouches.
Upon the anterior wall of the spiracle demonstrate a row of small vestigial gills; being supplied with arterial instead of venous blood they form what is termed a pseudobranch.