Underlying the skin of Ammocoetes in the branchial region is situated the sheet of longitudinal body-muscles, divided into a series of segments or myotomes, which forms the somatic muscles so characteristic of all fishes. This muscular sheet is depicted on the left-hand side of Fig. 54. It does not extend over the lower lip or over that part in the middle line where the thyroid gland is situated. In these parts a sheet of peculiar tissue known by the name of muco-cartilage lies immediately under the skin, covering over the thyroid gland and lower lip. The somatic muscular sheet with the superjacent skin can be stripped off very easily owing to the vascularity and looseness of the tissue immediately underlying it. When this is done the branchial basket-work comes beautifully into view as is seen on the right-hand side of Fig. 54. It forms a cage within which the branchiæ and their muscles lie entirely concealed. This is the great characteristic of this most primitive form of the branchial cartilaginous bars and distinguishes it from the branchial {128}bars of other higher fishes, in that it forms a system of cartilages which lie external to the branchiæ--an extra-branchial system. This branchial basket-work is simpler in Ammocoetes than in Petromyzon, and its actual starting-point consists of a main transverse bar corresponding to each branchial segment; from this transverse bar the system of longitudinal bars by which the basket-work is formed has sprung. These transverse bars arise from a cartilaginous longitudinal rod, situated close against the notochord on each side. These rods may be called the subchordal cartilaginous bands (Fig. 53), and, according to the observations of Schneider and others, each subchordal band does not form at first a continuous cartilaginous rod, but the cartilage is conspicuous only at the places where the transverse bars arise. In the youngest Ammocoetes examined by Schaffer, he could find no absolute discontinuity of the cartilage except between the first two transverse bars, but he says that the thinning between the transverse bars was so marked as to make it highly probable that at an earlier stage there was discontinuity. The whole system of branchial bars and subchordal rods is at first absolutely disconnected from the cranial system of trabeculæ and parachordals, and only later do the two systems join. [Illustration: FIG. 54.--VENTRAL VIEW OF HEAD REGION OF AMMOCOETES. _Th._, thyroid gland; _M._, lower lip, with its muscles.]