Further details in regard to this, the last revolution in Molluscan classification, will be found in the article Polyzoa.
The Molluscan ctenidium is typically a plume like structure, consisting of a vascular axis, on each side of which is set a row of numerous lamelliform or filamentous processes.
Increase of knowledge has now, however, established the conclusion that the agreement of structure supposed to obtain between Polyzoa and true Mollusca is delusive; and accordingly they, together with the Brachiopoda, were removed from the Molluscan phylum by Lankester in his article in the 9th edition of this work (on the which present article is based).
The latter are represented in the Upper Cambrian formations, together with Lamellibranchia and Gastropoda, and there are no earlier Molluscan fossils than these.
It now passes to the veliger phase, a definitely molluscan form, in which the disproportion between the area in front of the ciliated circlet and that behind it is very greatly increased, so that the former is now simply an emarginated region of the head fringed with cilia.
In the hinder part of the foot (not shown in any of the diagrams) is the opening of a large mucusforming gland very often found in the Molluscan foot.
These and other early monographs on the Tertiary shells of the Paris basin, of the environs of Bordeaux, and of the sub-Apennine formations of Italy, brought out the striking distinctness of these faunas from each other and from other molluscan faunas.
O, Mouth; other letters as in a totally distinct series of functional gills, which are not derived from the modification of the typical molluscan ctenidium.
Some are perforating algae and burrow into the substance of molluscan shells, in company with certain Green and Blue-green Algae.
So far the young mollusc has not reached a definitely molluscan stage of Xviji.