So far the young mollusc has not reached a definitely molluscan stage of Xviji.
It is held by some morphologists that the mollusc body is unsegmented, and therefore is to be compared to a single segment of a Chaetopod or Arthropod.
Still, Lake Baikal has a seal (Phoca vitulina, Phoca baikalensis of Dybowski) quite akin to the seals of Spitsbergen, marine sponges, polychaetes, a marine mollusc (ancilodoris), and some marine gammarids.
De Geer (named from the remains of the mollusc Ancylus fluviatilis), which is supposed to have covered an area of about 220,000 sq.
To this he added the account of Thule (which he placed six days' voyage north of Britain) and the adjoining regions, in which there was no longer any distinction between air, earth and sea, but a kind of mixture of all three, resembling the gelatinous mollusc known as pulmo marinus, which rendered all navigation and progress in any other mode alike impossible.
A medusa with a remarkable habit of life is Mnestra parasites, which is parasitic on the pelagic mollusc Phyllirrhoe, attaching itself to the host by its subumbral surface; its tentacles, no longer required for obtaining food, have become rudimentary.
They invariably disappear before the adult stage is reached, but their presence in the larva is evidence that the ancestral mollusc possessed a pair of true nephridia quite distinct from the coelomic excretory organs, which are so characteristic of existing forms in the adult condition.
This circlet of gill-lamellae led Cuvier to class the limpets as Cyclobranchiata, and, by erroneous identification of them with the series of metamerically repeated ctenidia of Chiton, to associate the latter mollusc with the former.
COCKLE, in zoology, a mollusc (Cardium) of the class Lamellibranchia.
That pagurids must have the usually soft pleon or abdomen protected by the shell of a mollusc is now known to be subject to a multitude of exceptions.