The excretory system is composed of two segmental ducts, each connected in front with a well-developed pronephros (head-kidney), with about five ciliated funnels opening into the pericardial region of the body cavity. The segmental ducts in the larvæ open behind into the cloacal section of the alimentary tract. [FIG. 47. HEAD OF A LARVA OF PETROMYZON SIX WEEKS OLD. (Altered from Max Schultze.) _au.v._ auditory vesicle; _op._ optic vesicle; _ol._ olfactory pit; _ul._ upper lip; _ll._ lower lip; _or.p._ papillæ at side of mouth; _v._ velum; _br.s._ extra branchial skeleton; 1-7. branchial clefts.] The development of the larva takes place with considerable rapidity. The yolk becomes absorbed and the larva becomes accordingly more transparent. It generally lies upon its side, and resembles in general appearance and habit a minute Amphioxus. It is soon able to swim with vigour, but usually, unless disturbed, is during the day quite quiescent, and chooses by preference the darkest situations. It soon straightens out, and, with the disappearance of the yolk, the tail becomes narrower than the head. A large caudal fin becomes developed. When the larva is about twenty days old, it bears in most anatomical features a close resemblance to an Ammocoetes; though the histological differences between my oldest larva (29 days) and even very young Ammocoetes are considerable. The mouth undergoes important changes. The upper lip becomes much more prominent, forming of itself the anterior end of the body (fig. 47, _ul_). The opening of the nasal pit is in this way relatively thrown back, and at the same time is caused to assume _a dorsal position_. This will be at once understood by a comparison of fig. 43 with fig. 47. On the inner side of the oral cavity a ring of papillæ is formed (fig. 47, _or.p_). Dorsally these papillæ are continued forward as a linear streak on the under side of the upper lip. A communication between the oral cavity and the branchial sack is very soon established. The gill pouches gradually become enlarged; but it is some time before their small external openings are established. Their walls, which are entirely lined by hypoblast, become raised in folds, forming the branchial lamellæ. The walls of the head cavities between them become resolved into the contractors and dilators of the branchial sacks. The extra-branchial basketwork becomes established very early (it is present in the larva of 6 millimetres, about 9 days after hatching) and is shewn in an older larva in fig. 47, _br.s_. It is not so complicated in these young larvæ as in the Ammocoetes, but in Max Schultze's figure, which I have reproduced, the dorsal elements of the system are omitted. On the dorsal wall of the branchial region a ciliated ridge is formed, which may be homologous with the ridge on the dorsal wall of the branchial sack of Ascidians. It has been described by Schneider in Ammocoetes.