Remove the skin from the roof of the pharynx. This exposes four pairs of _efferent branchial arteries_ bringing blood from the gills and uniting in pairs to form the _dorsal aorta_. Follow each vessel of the left side out to its gill-cleft. At the dorsal end of the gill-cleft it divides into a large posterior and small anterior branch. These respectively pass along the posterior and anterior demibranchs of the gill pouch, receiving fine branches from the gill lamellae, and finally unite again at the ventral end of the gill pouch. Thus a complete loop is formed around the branchial cleft. The posterior branch of each efferent artery and the anterior branch of the succeeding one are united by several short vessels. The efferent artery of the last demibranch possesses only these connections with the branch next anterior to it, and none with the aorta directly. From the ventral ends of the efferent loops small vessels pass toward the midline to unite with a longitudinal artery, the _hypobranchial artery_, which will be traced farther a little later in the dissection.
In Eugaleus the dorsal aorta extends forward beyond the union of the first pair of efferent branchials and then divides into small right and left branches which pass forward and outward to unite with the common carotid arteries.
A _common carotid artery_ leaves the dorsal end of each anterior efferent branchial loop, passing forward and inward. At the level of the spiracles it divides into _external_ and _internal carotids_; the internal carotid unites with its fellow of the opposite side and enters the skull. The external carotid arteries run outward and forward around the eyes and are distributed to the regions of the mandible and snout. Do not, at present, trace them beyond the posterior edge of the eye.
Another vessel arises from the middle of the anterior side of the first efferent branchial loop and runs forward to the spiracle, where it ends in the capillaries of the pseudobranch. This is the _afferent hyoidean artery_. The term pseudobranch is used for the branchial lamellae of the spiracle rather than demibranch because of the arterial blood supply of this organ.
Immediately after uniting the internal carotids divide and diverge, forming an X-shaped figure. Each anterior limb of the X again divides into two branches. The lateral branch passes to the ventral surface of the skull; it presently gives off an anterior twig (_ophthalmic artery_) which enters the eye. It then passes on as the _efferent hyoidean artery_ to the pseudobranch. The inner of the two branchs mentioned above passes on as the _internal carotid, sensu strictu_, and is distributed to the brain.