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	Class Name Number of Species Percent % Gastropoda ca. 65.000 - 80.000 ~ 76% Bivalvia ca. 20.000 ~ 21% Cephalopoda ca. 900 ~ 1% Scaphopoda ca. 900 ~ 1% Monoplacophora ca. 25 < 1% Polyplacophora ca. 1.000 ~ 1% Solenogastres ca. 300 < 1% Caudofoveata ca. 150 < 1% Mollusca ca. 85.000 - 100.000		Diagram: Enlarge Diagram!
	Number of species in Mollusca, displayed by classes, including percentage. Sources: WoRMS: MolluscaBase eds. (2025): Mollusca LINNAEUS, 1758.

Wenz, W. (1940): Ursprung und frühe Stammesgeschichte der Gastropoden. Archiv für Molluskenkunde 72: 1-110.
MolluscaBase eds. (2025). Monoplacophora Odhner, 1940.

Introduction

Dorsal view of Neopilina galatheae. Below: Lateral view. Source: Monoplacophora (Univ. Mich. Mus. Zool. 1995).

In 1952, during a Danish oceanographic expedition with the research ship Galathea off the western coast of Costa Rica in Central America, a new mollusc species was found, which was about to decisively change the modern understanding of mollusc biology ( Malacology).

Several inconspicuous limpet-like creatures about 3 cm in size had been collected from a depth of 3750 m. At first, the specimens were kept in storage, to be later examined by specialists. The results of the examination, published by the Danish zoologist H. Lemche 1957 in a paper, surprised the scientific world:

Not only the small molluscs definitively were not limpets: in contrary, they belonged to a group of molluscs so far only known as fossils and extinct since the Devonian period, 375 million years ago. After a fossil genus from that group, Pilina, Lemche called the newly discovered mollusc Neopilina - the new Pilina.

Lindberg, D.R. (2009): "Monoplacophorans and the origin and relationships of mollusks".  Evolution: Education and Outreach. 2 (2): 191–203. (PDF).
Geological Time Periods (Time Table).

Special Characters

Looking at Neopilina from a dorsal point of view, it apparently does look like a limpet: A cup shaped shell covers the entire dorsal side of the animal. In one specimen, even rests of the primordial spire could be found, so Neopilina's shell seemed to be coiled like that of a gastropod.

But from a closer point of view, Neopilina was not that inconspicuous: The shell tip points to the front, not to the side or to the rear like in all gastropods. The only known extant mollusc with a shell coiled to the front is Nautilus, a cephalopod. Cross-sections through the shell of Neopilina showed it to be a true mollusc shell with the known tripartite vertical build of the wall. Like in other molluscs, the shell is produced by cells in the dorsal epidermis, the mantle (pallium).

Schematic view of the ventral view of Neopilina. Source: Biodidac, modification: R. Nordsieck.

Schematic view of the internal organs of Neopilina. Yellow: Neural System; Orange: Exkretion Organs; Red: Heart and Gills; Dark Red: Musculature.

Now looking at the ventral side of a Neopilina, it becomes obvious that the similarity to a limpet-like gastropod is, in fact, only superficial and only regarding the shell. Neopilina does not have a head, only a slightly protruding mouth opening, behind which there are several mouth tentacles. The foot is round and most likely only enables the molluscs to crawl very slowly. On either side of the foot, five comb gills (ctenidia), typical for molluscs, are visible. Apart from the plurality of gills, Neopilina is also different from snails in that it does not have a pallial cavity like most higher molluscs do, but a lateral and caudal mantle groove between mantle and foot. In this groove is where the ctenidia are placed. Between the gills, the equally serial retractor muscles of the foot are visible, as well as the plurally built nephridia or excretory organs. Those, arranged serially, are usually only known from annelid worms (Annelida).

Neopilina sp., animal crawling to the right. Source: Sigwart, J.D. et al. (2019) (Link).

The originally paired gonads of Neopilina are joined together. However, the resulting single gonad retains two separate exiting ducts to release eggs and sperm into the water: the same situation as in chitons (Polyplacophora). Neopilina is dioecious with separate sexes; it is assumed that fertilization takes place externally.

The neural system of Neopilina also looks rather ancient: There are two lateral neural pathways: a pleural nerve (mantle nerve) and a pedal nerve (foot nerve). They join at the animal's posterior end to each form a closed neural ring. Between them, both neural rings are linked by transversal connections (connectives), the result being a rope-ladder shaped nervous system, as it is known from arthropods. Neopilina has no visible neural knots (ganglia).

Summary

As a summary, Neopilina displays highly evolved (derived) characters, but also ancient ones. The shell on one hand does qualify Neopilina as a highly developed mollusc like other shell-bearing ones (Conchifera), such as gastropods and bivalves, as opposed other molluscs, such as chitons (Polyplacophora) and the shell-less molluscs ("Aplacophora"), such as solenogasters and caudofovetans, which have not developed a complete shell yet, but instead rely on calcareous scales or spicules for their protection. On the other hand, in Neopilina's anatomy, there are also several characters appearing rather ancient, more resemblant that of the just mentioned Polyplacophora, Caudofoveata and Solenogastres, from which Neopilina remains distinctly different due to its derived shell.

After Lemche's publications in 1957 the scientific world generally assumed to have found a missing primordial mollusc, Neopilina thus being a living fossil. What contradicts this, is especially the distinctly derived conchiferan shell, so today it is assumed that the other Conchifera, as well as the Monoplacophora class with Neopilina and its relatives extinct since the Devonian period, have evolved from common ancestors.

The Monoplacophora are presumed to have split off the evolutionary line early, which explains the combination of derived conchiferan characters and ancient mollusc characters. However, the annelid-type characters of these ancient molluscs (such as the rope-ladder type nervous system and the serial nephridia) today are deemed to be derived or newly developed.

Systematics

Today, in the class Monoplacophora, apart from 4 fossil orders, only one extant order, Neopilinida with about 25 extant species, is listed.

Class: Monoplacophora Odhner, 1940

Feeding tracks of Neopilina sp. on "Utu" sea mountain near American Samoa.

Location of the "Utu" sea mountain.

Neopilina sp. on "Utu" sea mountain in a depth of 3837 m. Source: Sigwart, J.D. et al. (2019) (Link).

Order: Cyrtonellida (fossil)

Order: Neopilinida Lauterbach, 1983
ÜberFamily: Neopilinoidea Knight & Yochelson, 1958
Family: Neopilinidae Knight & Yochelson, 1958
Subfamily: Neopilininae Knight & Yochelson, 1958
Genus: Adenopilina Starobogatov & Moskalev, 1987
Genus: Laevipilina J. H. McLean, 1979
Genus: Monoplacophorus Moskalev, Starobogatov & Filatova, 1983
Genus: Neopilina Lemche, 1957: Neopilina galatheae Lemche, 1957
Genus: Vema A. H. Clarke & Menzies, 1959
Subfamily: Veleropilininae Starobogatov & Moskalev, 1987
Genus: Micropilina Warén, 1989
Genus: Veleropilina Starobogatov & Moskalev, 1987

Order: Sinuitopsida (fossil)

Subclass: Tergomya (fossil)

Order: Tryblidiida (fossil)

Source: MolluscaBase eds. (2025). Monoplacophora Odhner, 1940.

Links und Literatur

• Wikipedia: Neopilina, Monoplacophora.
• Sigwart, J. D.; Wicksten, M. K.; Jackson, M. G. et al. (2019): Deep-sea video technology tracks a monoplacophoran to the end of its trail (Mollusca, Tryblidia). Mar Biodiv 49, 825–832. (Link).

Latest Change: 01.10.2025 (Robert Nordsieck).