(Tryblidiacea Wenz 1938, in Lemche 1957, *Monoplacophora* Odhner 1940, partim, etc.)
Dorsal view of Neopilina galatheae. Below: Lateral view.
Source: Class Monoplacophora (Univ. Mich. Mus. Zool. 1995).
In 1952, during a Danish oceanographic expedition with the research vessel Galathea off the Western coast of Costa Rica a new mollusc was found, which would fundamentally change the comprehension of mollusc biology.
The inconspicuous limpet-like creatures had a shell length of about 3 cm (2.2 in.) and were dredged from a depth of 3750 metres (11250 ft.). First the samples were conserved to be examined later by Danish specialists. The examination results published by the Danish zoologist H. Lemche in 1957 surprised the scientific world: Not only the creature definitively was not a limpet, instead it belonged to a group of molluscs only known as fossils from the Cambrian period, several hundred million years ago. After a fossil species from that group, Pilina, Lemche named the newly discovered mollusc Neopilina , the new Pilina.
Looking at a Neopilina from above it actually seems to resemble to a limpet, having a limpet-like dorsal shell. One specimen was found with rests of a primordial spire, so the shell of a juvenile Neopilina is coiled like that of a gastropod.
But, and that is one of the special things about Neopilina: The shell tip points towards the head, not towards the tail or to the side, like in gastropods. The only known recent mollusc with a shell coiled forward is Nautilus, a cephalopod. Cross-sections through a Neopilina shell proved it to be a real mollusc shell with the customary three vertical layers. Also, like in other molluscs, the shell is built by a dorsal epidermis, the mantle, or pallium.
Schematic illustration of Neopilina 's ventral side.
Source: Biodidac; changes by: R. Nordsieck.
Looking at the ventral side of a Neopilina , the resemblance to a limpet becomes clearly superficial. Neopilina does not have a head, only the mouth opening prolonged a little bit, with some mouth tentacles next to it.
The foot of Neopilina is round and most probably enables Neopilina to a very slow crawling motion. On either side of the foot there are five comb-gills (Ctenidia) typical for molluscs. In addition to the large number of gills, Neopilina is also different to gastropods in not having a pallial cavity, like most of the higher molluscs, but a pallial groove running between mantle and foot on the side and on the tail end (laterally and caudally). In this groove, the gills are position in a serial manner. An anatomical examination reveals the retractor muscles of the foot also positioned serially, as well as several nephridia, organs of excretion, in this serial type usually only found among segmented worms (Annelida).
Schematic illustration of Neopilina 's internal
organization. Yellow: Nervous system; Orange:
Excretion organs; Red: Heart and Gills; Dark red:
Source: Biodidac, further processing: R. Nordsieck.
The pair of gonads in Neopilina are joined to form a single gonad, which however releases germ cells into the water by two separate outgoing ducts. Neopilina has separate sexes; it is assumed that fertilisation takes place externally in the water.
Neopilina 's nervous system also appears very primordial: Two neural pathways run on either side: One pleural nerve (mantle nerve) and one pedal nerve (foot nerve). They join at the creature's caudal end, making a closed neural ring. Between each other, the two neural pathways are also linked by crosswise connections (connectives), so the neural system resembles an arthropod's rope ladder style neural system. Neopilina lacks neural knots (ganglia).
Altogether Neopilina shows characters advanced as well as primordial:
After Lemche's publications in 1957, the scientific world generally assumed to have found the primordial mollusc as such. Proof against this assumption mainly is the advanced Conchifera shell, so the current theory assumes Neopilina, with its extinct predecessors from the Cambrian of the Tryblidia class, to have evolved from common ancestors with the all Conchifera.
The Tryblidia's evolutionary pathway seems to have diverged at an early time from the main evolutionary line, which would explain the combination of the Conchifera characters and the primordial characters of ancient molluscs. In those ancient molluscs, the characters resembling the Annelida (the rope ladder style nervous system and the serial nephridia) are today assumed to be characters newly evolved in phylogenesis.
Today the class Tryblidia is estimated to comprise about 20 recent species in one recent order, Tryblidia with two recent families, Neopilinidae and Micropilinidae. The obsolete term Monoplacophora (one-shell molluscs) only incompletely describes a part of the class.