At a very young stage the larva has the form ([fig. 142]) of a flattened sphere, with a small conical knob at the posterior extremity.
At the equator are situated two parallel ciliated bands[137], between which lies the ventrally placed mouth (m). The more conspicuous ciliated band is formed of a double row of cilia, and is situated in front of the mouth. The thinner ciliated band behind the mouth appears to be absent in the American species.
The mouth leads into an œsophagus, 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-œsophageal 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.
