The condition usually spoken of as a " proboscis " appears to be derived from the condition of a simple rostrum (having the mouth at its extremity) by the process of incomplete introversion of that simple rostrum.
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 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.
The probability appears to be against an actual introversion of the appendage and its lamellae, as was at one time suggested by Lankester.
Each cansists of an eversible hollow tentacle provided with hooklets and capable of introversion within a mem The excretory organs consist of flame-cells, richly convoluted canaliculi, and a pair of longitudinal canals leading to the exterior by one or more pores.