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.
The general structure of the Molluscan intestine has not been sufficiently investigated to render any comparison of this structure of Patella with that of other Mollusca possible.
The eyes of the limpet deserve mention as examples of the most primitive kind of eye in the Molluscan series.
It corresponds to the right of the two primitive ctenidia in the untwisted archaic condition of the molluscan body, and does not project freely into the branchial cavity, but its axis is attached (by concrescence) to the mantle-skirt (roof of the branchial chamber).
As in other Molluscan groups, we find a wide variation in the early process of the formation of the first embryonic cells, and their arrangement as a diblastula, dependent on the greater or less amount of food-yolk which is present in the egg-cell when it commences its embryonic changes.
Accordingly, the shell of Aplysia must not be confounded with a primitive shell in its shell-sac, such as we find realized in the shells of Chiton and in the plugs which form in the remarkable transitory " shell-sac " or " shell-gland " of Molluscan embryos (see figs.
Thus the renal organ of Aplysia is shown to conform to the Molluscan type.
36) to the adult form has not been properly observed, and many interesting points as to the true nature of folds (whether parapodia or mantle or velum) have yet to be cleared up by a knowledge of such development in forms like Tethys, Doris, Phyllidia, &c. As in other Molluscan groups, we find even in closely-allied genera (for instance, in Aplysia and Pleurobranchidium, and other genera), the f operculum.
1, The lamelliform sub-pallial gills, which (as in Patella) replace the typical Molluscan ctenidium.
Curiously enough, however, they differ from the cephalic Molluscan eye in the fact that, as in the vertebrate eye, the filaments of the optic nerve penetrate the retina, and are connected with the re surfaces of the nerve-end cells nearer the lens instead of with the opposite end.
The Miocene beds are also marine and are characterized by an abundant molluscan fauna.
The essential Molluscan organs are, with these exceptions, uniformly well developed.
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.
Vaughan Thompson is again that which is primarily connected with the history of a Molluscan class.
Henri Milne-Edwards in 1844 demonstrated the affinities of the Polyzoa with the Molluscan class Brachiopoda, and proposed to associate the three classes Brachiopoda, Polyzoa and Tunicata in a large group " Molluscoidea," co-ordinate with the remaining classes of Cuvier's Mollusca, which formed a group retaining the name Mollusca.
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).
Further details in regard to this, the last revolution in Molluscan classification, will be found in the article Polyzoa.
The latter are represented in the Upper Cambrian formations, together with Lamellibranchia and Gastropoda, and there are no earlier Molluscan fossils than these.
So far the young mollusc has not reached a definitely molluscan stage of Xviji.
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.
This is the primitive shell-sac discovered by Lankester in 1871, and shown by him to precede the development of the permanent shell in a variety of molluscan types.
The ctenidia, it will be observed, have not yet been mentioned, and they are indeed the last of the characteristic Molluscan organs to make their appearance.
Hubrecht in 1881 of a typical molluscan radula and odontophore in a new genus Proneomenia, allied to Neomenia, showed that the whole group belonged to the Mollusca.
Each gill has the structure of a typical molluscan ctenidium, consisting of an axis bearing an anterior and posterior row of filaments or lamellae.
Some are perforating algae and burrow into the substance of molluscan shells, in company with certain Green and Blue-green Algae.
The molluscan fauna is fairly rich, and insect fauna much more so, even in the north.