It is clear that, if we start from the condition of full eversion of the tube and watch the process of introversion, we shall find that the pleurecbolic variety is introverted by the apex of the tube sinking inwards; it may be called acrembolic, whilst conversely the acrecbolic tubes are pleurembolic. Further, it is obvious enough that the process either of introversion or of eversion of the tube may be arrested at any point, by the development of fibres connecting the wall of the introverted tube with the wall of the body, or with an axial structure such as the oesophagus; on the other hand, the range of movement of the tubular introvert may be unlimited or complete.
The acrembolic proboscis or frontal introvert of the Nemertine worms has a complete range.
(Lankester.) A, Simple introvert completely introverted.
D, E, A similar simple introvert in course of eversion by the forward movement, not of its sides, but of its apex, as in the proboscidean Rhabdocoels =acrecbolic.
F, Acrecbolic (= pleurembolic) introvert, formed by the snout of the proboscidiferous Gastropod.
The introvert is not a simple one with complete range both in eversion and introversion, but is arrested in introversion by the fibrous bands at c, and similarly in eversion by the fibrous bands at b.
- A, Triton variegation, to show the proboscis or buccal introvert (e) in a state of eversion.
A, Siphonal notch of the shell e, Everted buccal introvert (prooccupied by the siphonal boscis).
Tentacles retractile into an introvert ("tentacle-sheath").
- Zooecia with soft uncalci fled' walls, the external part of the introvert being closed during retraction by a membranous collar.
In some Gymnolaemata, polypides which develop an ovary possess a flask-shaped "intertentacular organ," situated between two of the tentacles, and affording a direct passage into the introvert for the eggs or even the spermatozoa developed in the same zooecium.
At the tip of the introvert the mouth opens, and is surrounded in Sipunculus by a funnel-shaped, ciliated lophophore (figs.
Their function is to retract the introvert, which is protruded again by the contraction of the circular muscles of the skin; these, compressing the fluid of the body-cavity, force forward the anterior edge of the introvert.
(vi.) Aspidosiphon, with 19 species, is easily distinguished by a calcareous deposit and thickened shield at the posterior end and at the base of the introvert, which is eccentric. (vii.) Cloeosiphon has a calcareous ring, made up of lozenge-shaped plates, round the base of its centric introvert.
- Sipunculus nudus, L., with introvert and head fully extended, laid open by an incision along the right side to show the internal organs, X 2.