Lophotrochozoa
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| Halanych et al., 1995 |
The Lophotrochozoa (Lophotrochozoa; gr. Λόφος lophos - helmbush , trochos - wheel, tire, zoon - animal) are primarily a molecular-genetically determined supernary of animals within the trunk group of the original mouths (protostomia).
The Lophotrochozoa primarily comprise worm-like organisms, but the molluscs (Mollusca) and armpods (Brachiopoda) also belong here. The most complicated morphological structures are found in the cephalopods within the molluscs.
Systematics
The Lophotrochozoa include two larger animal groups that are usually recognized by their morphology , the Lophophorata (also misleadingly called tentaculata, especially in the German and Russian-speaking areas) and the Trochozoa, which from a systematic point of view can only be considered as stages of development, since they do not have any Monophyla represent.
The fact that the Lophotrochozoa themselves represent a closed community of descent is widely recognized today, but is just as controversial. The traditional competitive hypothesis is the Articulata concept, which is mainly based on morphological findings. From this point of view, a close relationship between annelids and arthropods ( insects , arachnids and others) is assumed, which is evidenced by the external segmentation shared by the two groups . Neither the results of molecular genetics nor a study of the developmental stages confirm such a grouping: the arthropods in particular do not have any intermediate stages related to the Trochophora larvae (see below). On the problem of dissolving the traditional articulata concept, see also Ecdysozoa .
The Lophophorata
The Lophophorata comprise the following animal phyla :
- The arm pods (Brachiopoda), which superficially resemble the mussels (Bivalvia), but have a fundamentally different structure, today form a group with almost 340 known species, which is no longer particularly successful, but was particularly widespread and species-rich in the ancient times (Paleozoic). The fertilized eggs first develop into a ciliated larva, which then develops into an adult animal.
- The bog animals (Ectoprocta) are also known as Bryozoa and are known fossil like the arm pods. They comprise about 5000 species and got their name because they often live in large colonies that look like a cushion of moss on the surface. They do not develop into the adult form immediately, but rather go through a stage beforehand called the Cyphonautes larva.
- The horseshoe worms (Phoronida) are a very small group of worm-like organisms. They owe their name to the horseshoe-shaped form of their tentacle organ called lophophore. They also develop into an adult animal via an intermediate stage called actinotrocha larvae.
In addition to similar larval stages the Lophophorata show something else in common: the lophophore , a specialized tentacle apparatus, after which they are named. The Lophophor is used for nutrition and is often ring-shaped, sometimes also horseshoe-shaped or wound up spirally composed of the tentacles. These are hollow on the inside as a continuation of the body cavity ( coelom ) and ciliate on the outside in such a way that a synchronized beat pattern creates a water flow that leads food particles to the mouth located inside the tentacle ring. The anus is located outside of this ring.
Because of the relative complexity of the Lophophors, the monophyletic position of the Lophophorata was assumed for a long time.
The Trochozoa
Within the Trochozoa one differentiates the following animal phyla:
- The molluscs (Mollusca) are one of the best-known groups of invertebrates: They include a large number of well-known animals with the mussels (Bivalvia), snails (Gastropoda) and cephalopods (cephalopods), which include, for example, squids and cuttlefish . With 110,000 known species, they form one of the most species-rich phyla. The first fossil finds can already be found in the ancient times (Paleozoic). The adult animals develop via a so-called Veliger larva.
- The annelid worms (Annelida) form a group of animals enclosing around 9000 species, which are characterized by their outer division into segments, which continue inward. Because of this segmentation, it has long been assumed that they are closely related to the arthropods , which include arachnids and insects , for example . Today, however, the majority of people assume that the similarities are only superficial. Annular worms, which also include the common earthworm ( Lumbricus terrestris ), but which also include the leeches (Hirudinea), develop from a trochophora larva, which gives the trochozoa its name. They are already known as fossils from the so-called Ediacara fauna of the Proterozoic . The hedgehog worms (Echiura), which used to be regarded as a separate strain, are also classified within the annex worms . They have a short, mostly sack-shaped trunk and a proboscis called proboscis. A distinction is made between about 150 species, which sometimes show extremely extreme reproductive conditions; sometimes the female is longer than a meter, while the male is only a few millimeters. The relationship with the annelid worms is considered to be almost certain due to the developed trochophoral larva; they are often also viewed as taxon outside of the annelid worms. This also applies to the 300 or so species that belong to the injection worms (Sipuncula). They are characterized by a trunk structure called an introvert, which can usually be pulled completely into the trunk. The mouth opening is often surrounded by a ring of tentacles with lashes. Squirtworms have been found at a depth of 7,000 meters. In many species there is a trochophoral larva as a developmental stage, sometimes there is also a second larval state, the Peganosphaera larva.
- Cordworms (Nemertea) are characterized by a special structure, the proboscis. This is a kind of proboscis device that is used by many of the approximately 900 species for catching prey or for defense. The cordworm Lineus longissimus is one of the largest invertebrates with a length of up to 30 m. Here, too, the fertilized egg cell often develops into an adult worm via an intermediate stage called the Pilidium larva.
Characteristic of all Trochozoa is the intermediate stage known as trochophora larva in the annelid worms: These larvae, adapted to swimming in the open sea, are characterized by two eyelash bands looped around the middle of the body and a tuft of longer flagella at the "head end" and come in this form in all six Trochozoa strains. From them the great variety of adult animals develops, which as leeches, cordworms, snails, mussels or cephalopods can often look very different. The close relationship has also been confirmed by molecular genetic results.
literature
- Wilfried Westheide, Reinhard Rieger (Ed.): Special Zoology - Part 1: Unicellular and invertebrate animals. 2nd Edition. Elsevier, Spektrum Akademischer Verlag, Munich 2007, ISBN 978-3-8274-1575-2 .
- R. Podsiadlowski: Phylogeny and mitochondrial gene order variation in Lophotrochozoa in the light of new mitogenomic data from Nemertea. In: BMC Genomics. 10, 2009, p. 364. doi: 10.1186 / 1471-2164-10-364
- G. Giribet: Assembling the lophotrochozoan (= spiralian ). tree of life. In: Philos. Trans. R. Soc. Lond., B, Biol. Sci. 363, 1496, April 2008, pp. 1513-1522. doi: 10.1098 / rstb.2007.2241