Fig. 129. Two stages in the development of Pedicellina.
(After Hatschek.)
oe. œsophagus; ae. archenteron; an.i. anal invagination; f. fold of epiblast; f.g. ciliated disc; x. problematical body derived from hypoblast (probably a bud).
This prominence is perhaps equivalent to the epistome of the Phylactolæmata and the disc-like organ of Rhabdopleura, which Lankester has compared to the molluscan foot[120].
Very shortly after the first formation of the vestibule there appears at the opposite end of the larva a thickening of the epiblast, which soon becomes invaginated, and forms an eversible pit ([fig. 129] A and B, f.g.). Round its mouth there is formed a ring of stiff cilia ([fig. 130], f.g.). This organ is very possibly equivalent to the cement gland described by Kowalevsky in the adult Loxosoma. I shall speak of it as the ciliated disc.
The epiblast cells early secrete a cuticle.
The two mesoblast cells soon increase by division, and occupy the space between the alimentary canal and the body wall. They do not become divided into a splanchnic and somatic layer; but give rise to the interstitial connective tissue and muscles. From the mesoblast there is also formed, according to Hatschek, a pair of ciliated excretory canals, in the space between the mouth and anus ([fig. 130] B, nph.). The development of the nervous system has not been observed.
At a comparatively late stage in the development there is formed round the edge of the vestibule a ring of long cilia ([fig. 130] B, m.).