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_.