The introversible tube may be completely closed, as in the " proboscis " of Nemertine worms, or it may have a passage in it leading into a non-eversible oesophagus, as in the present case, and in the case of the eversible pharynx of the predatory Chaetopod worms. The diagrams here introduced (fig.
Supposing the tube to be completely introverted and to commence its eversion, we then find that eversion may take place, either by a forward movement of the side of the tube near its attached base, as in the proboscis of the Nemertine worms, the pharynx of Chaetopods and the eye-tentacle of Gastropods, or by a forward movement of the inverted apex of the tube, as in the proboscis of the Rhabdocoel Planarians, and in that of Gastropods here under consideration.
The acrembolic proboscis or frontal introvert of the Nemertine worms has a complete range.
B, The same, partially everted by eversion of the sides, as in the Nemertine proboscis and Gastropod eye-tentacle = pleurecbolic.
- The organ most characteristic of a Nemertine is without doubt the proboscis.
- The brain of a Nemertine, with its lobes and commissures.
Two pairs of invaginations of B the skin, which originally are called the prostomial and metastomial disks, grow round the intestine, finally fuse together, and form the skin and mus- cular body-wall of the future Nemertine, which afterwards becomes ciliated, frees itself from the pilidium investment and develops into the adult worm without further metamorphosis.
In accordance with these more sedentary habits during the first phases of life, the characteristic pilidium larva, which is so eminently adapted for a pelagic existence, appears to have been reduced to a close-fitting exterior layer of cells, which is stripped off after the definite body-wall of the Nemertine has similarly originated out of four ingrowths from the primary epiblast.
A, pilidium larva anus by the closure in of nemertine; B, Asplanchnapus the middle of such a slitschematized; C, a ploimal rotifer; like blastopore opening D, trochosphaera female (schematized into a sack-like stomach from Semper); E, veliger larva of mol as is seen in the larvae lusc; F, trochophore larva of annelid.