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 œsophagus.
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.
As the hind portion of the body becomes elongated the stomach extends into it, and gives rise to the mesenteron of the adult ([figs. 143], [144], and [145]). For a long time the anterior spherical dilated portion of the larva remains very large, consisting of a præ-oral lobe and a post-oral section. The two together may be regarded as constituting the head.
At a comparatively late stage a pair of tentacles arises from the front end of the præ-oral lobe ([fig. 146]), and finally the head becomes relatively reduced as compared with the body, and gives rise to the simple head of the fully formed worm ([fig. 148]). The two ciliated bands disappear, the posterior vanishing first. The ciliated band at the hind end of the body also atrophies; and just in front of it the ring of wart-like prominences used by the adult to attach itself becomes developed.