There is no reason in the actual significance of the word why the term " proboscis " should be applied to an alternately introversible and eversible tube connected with an animal's body, and yet such is a very customary use of the term.
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.
On the other hand, many which have a siphonate mantle-skirt are not provided with an introversible mouth-bearing al- B c..
(From Owen.) cylinder, but have a simple non-introversible rostrum, as it has been termed, which is also the condition presented by the mouth-bearing region in nearly all other Gastropoda.
One of the best examples of the introversible mouth-cylinder or proboscis which can be found is that of the common whelk (Buccinum undatum) and its immediate allies.