Any impulse from theory, simply as a spontaneous outgrowth of popular life.
The siphon is an incompletely tubular outgrowth of the mantle margin on the left side, contained in a corresponding outgrowth of the edge of the shell-mouth, and serving to conduct water to the respiratory cavity.
The formation of a massive body naturally involves the localization of the absorptive region, and the function of absorption (which in the simpler forms is carried out by the whole of the vegetative part of the mycelium penetrating a solid or immersed in a liquid substratum) is subserved by the outgrowth of the hyphae of the surface-layer of that region into rhizoids, which, like those of the Algae living on soil, resemble the root-hairs of the higher plants.
The mouth, with its jaws, forms a conical outgrowth which projects backwards, so that its apex lies beneath the prothorax.
From the broader portion of the planula an outgrowth arises which becomes the first tentacle of the cormus.