Single scarf

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Single scarf
Pilina unguis

Pilina unguis

Systematics
Over trunk : Lophotrochozoa (Lophotrochozoa)
Trunk : Molluscs (mollusca)
Sub-stem : Shell molluscs (Conchifera)
Class : Single scarf
Order : Single scarf
Scientific name of the  class
Monoplacophora
Lemche , 1957
Scientific name of the  order
Tryblidiida
Odhner in Wenz, 1940

The Einschaler (Monoplacophora), also Urmützenschnecken or Napfschaler called, occur at depths from 170 to 6500 meters with only 27 species on the bottoms of the seas. Fossil almost 100 species are described.

As the Greek name suggests, they have a flat back shell consisting of only one piece, in contrast to the shells of the beetle snails ( Polyplacophora ) consisting of eight or more parts and in contrast to the two-valve shell of the mussels . They are not limpets that belong to the class of snails, but a separate class of animals.

Systematics

Under the name Monoplacophora ( Odhner in Wenz, 1940) a group of different molluscs with cap-shaped housings was summarized. This mainly concerned the fossil representatives, in which mostly only morphological external characteristics could be examined. The group soon turned out to be polyphyletic . As early as 1965 this led to an attempt to give up the name Monoplacophora and replace it with the name Tergomya (Horný, 1965). Some of the fossil monoplacophores were separated in 1991 as the new class Helcionelloida . Today, under the class name Monoplacophora, a part of the mollusks is summarized to which the recent order (Tryblidiida) belongs, which is represented by two families, the Neopilinidae and the Micropilinidae .

Shells of these animals were already well known in fossil form from the Cambrian to the Devonian , but were assigned to snails (Gastropoda). In 1957, the first of the animals thought to be extinct since the Devonian , were described from a depth of 3570 meters. The species was scientifically named Neopilina galatheae , after the Danish research vessel Galathea that found it. The genus name refers to the fossil genus Pilina .

features

casing

The cup bowls belong to the group of shell molluscs (Conchifera) and in many ways have retained the original blueprint with a uniform housing. The housing is cup-shaped and rolled up to the front. Fossil shapes can also have highly conical housings. In today's forms, the shell is made of aragonite . In addition to aragonite, some fossil forms also store calcite in the shell ( Tryblidium ).

Soft tissues

The housing covers the entire body of the animal, which consists of a large foot, a mouth area and a casing groove surrounding these two structures .

In the mantle channel there are 3 to 6 pairs of so-called gills , depending on the species , but they are primarily used to generate the water flow and less for breathing; Some other organs are also set up in pairs, especially the excretory organs ( nephridial sacs ) and the sex glands ( gonads ). Also typical are the eight pairs of dorsoventral muscles and a pair of radular retractors, which usually leave clear marks on the inside of the housing and can also be easily identified in fossil form. The pairing of different organs, however, has rather surely only developed in the course of evolution and does not constitute evidence of a close relationship with the annelids ( Annelida ).

The Neopilinidae have an open blood vessel system and a well developed heart with two pairs of atria. Two aortas emanate from the ventricle and merge further forward to form an aorta. A peripheral sinus takes the oxygen-laden blood fluid ( hemolymph ) coming from the mantle groove and guides it into the two antechambers of the heart. From there it flows into the heart's ventricle and then into the two aortas that run forward from the heart, which reunite to form an aorta near the head and pour the hemolymph into the body. The blood fluid flows back to the jacket space via a foot sinus. The Micropilinidae do not have a heart.

With regard to the nervous system, there is tetraneuria, that is, two paired (= four) longitudinal cords emanate from the pharynx and are connected to one another by serial connections . However, they do not have real ganglia (nerve nodes). This is considered to be the plesiomorphic state within the mollusks. However, there are distinct cerebral ganglia and buccal ganglia in the head area. There is also a subradular ganglion. In terms of sensory organs, the paired statocysts should be mentioned, but eyes and osphradia are missing.

A jaw is poorly developed. The rasp tongue ( radula ) has 11 tooth elements per transverse row, the lateral teeth are hardened by the deposition of ferritin . The esophagus of the Neopilinidae shows enormous enlargements, which Neopilina originally misinterpreted as "dorsal coelom". The stomach has a ferment stalk, the intestine is spiraled several times and ends at the rear end of the animal centrally in the mantle groove.

Way of life

nutrition

All Monoplacophora species today live in the deeper waters of the open oceans. So far they have been dredged from 174 m to more than 6000 m water depth. Vema lives on phosphate tubers at a depth of around 174 to 388 m off the coast of southern California. However, the Paleozoic Einplatter seem to have lived mainly in shallow marine areas. The intestinal contents of various species today have been analyzed. It contained detritus and protozoa (diatoms, radiolarians). Sponge needles and shredded remains of sea urchin spines were found in the intestinal contents of another species . Presumably they graze the sediment surface and in doing so take up detritus and other organic residues that lie there.

Multiplication

Most Tryblidia are separate sexes, but very little is known about fertilization and development of the animals. The only 0.9 mm long Micropilina arntzi is hermaphrodite and takes care of brood in its mantle channel.

Phylogeny

The single shellfish are still considered by some authors to be the parent group of the shell molluscs (Conchifera). In this view, however, the single scarfers are paraphyletic. However, this view contradicts the observation that the first undisputed single shells (with several paired muscle impressions) only appear from the Upper Cambrian, while the oldest mussels, for example, also a subgroup of shell molluscs , already appear in the Lower Cambrian. In most of the classifications, the single shells are seen as the sister group of the other classes of recent shell molluscs.

Genera

The class currently (February 2010) includes an order with two families, the Neopilinidae family comprising seven genera and the Micropilinidae family only one genus. The distinction between the genera and species is mainly made according to shell, radula, number of gills and nephridia as well as the oral tentacles.

Web links

Commons : Einaler  - collection of images, videos and audio files

literature

  • Horný, Radvan 1963. On the systematic position of cyrtonelloids (Mollusca). Časopsis národního Muzea, oddil přírodovědný, 132: 90–93, Prague.
  • Lemche, Henning 1957. A new living deep-sea mollusc of the Cambro-Devonian class Monoplacophora. Nature, 179: 413-416, London.
  • Rozov, SN 1975. A new order of the Monoplacophora. Paleontological Journal, 9: 39-43, Washington.
  • Wingstrand, Karl Georg 1985. On the anatomy and relationships of recent Monoplacophora. Galathea Report, 16: 7-94, Leiden & Copenhagen.
  • Haszprunar, G. & Schaefer, K. 1997a. Monoplacophora. In: Harrsion, FW & Kohn, AJ (eds.): Microscopic Anatomy of Invertebrates. Vol. 6B: Mollusca II: 415-457. Wiley-Liss, New York.
  • Haszprunar, G. & Schaefer, K. 1997b. Anatomy and phylogenetic significance of Micropilina arntzi (Mollusca, Monoplacophora, Micropilinidae fam. Nov.) . Acta Zoologica (Stockholm) 77 (4): 315-334.
  • Haszprunar, G. 2008. Monoplacophora (Tryblidia) . In: Ponder, WF & Lindberg, DR (eds.): Phylogeny and Evolution of the Mollusca: pp. 97-104. Univ. Calif Press Berkeley.

Individual evidence

  1. ↑ Urania . Urania-Verlag, January 1, 1957, p. 236 ( books.google.de ).