The mouth leads into an oesophagus, and this into a globular stomach (_ol_), which is continuous with a rectum terminating by an anus (_an_) placed at the hind end of the posterior conical knob. The whole alimentary tract is ciliated. In the American form of larva there is a ring of cilia round the anus, which is developed at a somewhat later stage in the form observed by Hatschek. [FIG. 143 POLYGORDIUS LARVA. (From Alex. Agassiz.)] The position of the ciliated bands and the alimentary tract enables us to divide the embryo into three regions: a præ-oral region bounded by the anterior ciliated band, a gastric region in which the embryonic stomach is situated, and an abdominal region formed of the posterior conical portion, which by its subsequent elongation gives rise to the whole segmented portion of the future Polygordius. At the front end of the præ-oral lobe is situated the early formed supra-oesophageal ganglion (_sg_) (first noticed by Agassiz) in connection with which is a pair of eyes, and a ramified system of nerves. The ganglion is marked externally by a crown of cilia. [FIG. 144. POLYGORDIUS LARVA. (From Alex. Agassiz.)] The larval epidermis bears a delicate cuticula, and is separated by a considerable interval from the walls of the alimentary tract. The space between the two represents a provisional body cavity, which is eventually replaced by the permanent body cavity formed between the two layers of the mesoblast. It is doubtful when the replacement takes place in the head. It probably does so very early. The mesoblast is present in the usual form of two bands (_me.p_) (germinal streaks), which are anteriorly continued into two muscular bands which pass through the embryonic body cavity to the front end of the præ-oral lobe. Another pair of contractile bands passes from the same region of the præ-oral lobe to the oesophagus. There is no trace of the ventral nerve cord. The most remarkable organ of the larva is a paired excretory organ (_nph_) discovered by Hatschek. This is a ciliated canal with at first one and subsequently several funnel-shaped openings into the body cavity in front and an external opening behind. It is situated immediately anterior to the lateral band of mesoblast, and is parallel with, and dorsal to, the contractile band which passes off from this. It occupies therefore a position in front of the segmented region of the adult Polygordius. [FIG. 145. POLYGORDIUS LARVA. (From Alex. Agassiz.)] The changes by which this peculiar larval form reaches the adult condition will be easily gathered from an inspection of figs. 143-148. They consist essentially in the elongation of what has been termed the abdominal region of the body, and the appearance of a segmentation in the mesoblast; the segments being formed from before backwards, and each fresh segment being interpolated between the anus-bearing end of the body and the last segment.