Systematic classification:
Systematic classification:
Phylum: Mollusca (Molluscs)
Cuvier 1795
Class: Monoplacophora (Monoplacophorans)
Odhner 1940
Order: Tryblidiida Lemche 1957
Family: Neopilinidae Knight &
Yochelson 1958
Species: Neopilina galatheae Lemche 1957
Resource: Biodidac.
Neopilina is a mollusc about 35 mm (about 1.5 in) in size remotely
resembling a limpet, but which when observed more closely looks far else. The
limpet-like shell's point, by the way, looks front.
1952 the Danish Galathea-expedition made an astounding discovery. From a depth of 3000 m (9000 ft) an animal was brought to the surface, that seemed to be some kind of snail. In fact the only resemblance to the molluscs known so far was the typical mollusc shell. Further findings occurred during the following decades in the Red Sea, the Central Pacific, the Southern Atlantic and near the Antarctic in depths of between 2500 and 6500 m (7500 and 19500 ft).
Neopilina has got the typical mollusc body with the division in shell
and soft body. The latter again is divided into the dorsal epidermis (mantle
or pallium), the foot and the inner organs. Between foot and mantle there is
the pallial groove, that, similar to that of the chitons (Placophora),
serves the respiration and excretion. The situation of many organs, however,
makes think of another animal group, the calcareous shell will not suit.
Resource: Biodidac,
labeling by author.
Neopilina not only has one or two, but five or six gills, that are located in the pallial groove. As regularly situated are the dorsoventral (reaching from back to belly) retractor muscles of the foot, Neopilina possesses eight pairs of. Neopilina does not possess a kidney in the sense of the word, but six nephridia that open into the pallial cavity. Nephridia, however, are a sign for metamerous organisms like the segmented worms (Annelida), that in each segment possess the same number of equally built organs.
The nervous system of this astonishing organism is also very regularly built. From a ring system in the head region two pleural and two pedal nerves extend to meet at the tail end. Both nerves are linked laterally by several commissures. Neopilina does not possess any ganglia (nerve knots). That nervous system also reminds strongly of the rope ladder nervous system of the segmented animals (Articulata).
The sexual organs (gonads) are paired, large, and located centrally. The sexual
products (eggs and sperms) are excreted over two nephropores (efferent tubes
of the nephridia).
Resource: Biodidac,
labelling by author.
In biology, especially in evolutionary research, living fossils obtain outmost importance. Living fossils are thought to be recent (living today) members of elsewhere extinct groups of organisms. Examples for living fossils are Nautilus among the cephalopods, Neopilina galatheae among the primitive molluscs, and the horseshoe crab (Limulus polyphemus), among the crustaceans.
For a major period of time Neopilina, being a living fossil, served as a model organism for a possible archaic mollusc. Taking into account fossil records two evolutionary lines were distinguished, one of which lead from archaic molluscs like Neopilina to ancestors of the recent scaphopods and bivalves, the other leading to the snails (Gastropoda) and squid (Cephalopoda).
Today the characteristics of monoplacophorans which had first been thought to be ancient, have been classified as modern, meaning later evolved. Thus the status of Neopilina remains unclear.