Rhombozoa
| Rhombozoa | ||||||||||||
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Dicyema macrocephalum |
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| Scientific name | ||||||||||||
| Rhombozoa | ||||||||||||
| A. Krohn , 1839 | ||||||||||||
The Rhombozoa ( Gr .: Rautentiere ) are a small tribe of the animal kingdom.
Physique and characteristics
They are worm-shaped organisms that are 0.1 to a maximum of 9 mm long. Your body consists of only 10 to 40 cells and is cell-constant ( Eutelia ). It has neither body cavities nor differentiated organs. The body is two-layered, it consists of a large, generative axial cell with pockets, which is surrounded by a single-layer epidermis , the cells of which carry cilia . Pole cells form a front end called a "dome".
Life cycle and reproduction
Their life cycle is an alternation of asexually and sexually reproducing generations, i.e. a metagenesis . The first asexual generation is called nematogen . So-called axioblasts develop from their axial cell, from which so-called “vermiform” (worm-shaped) larvae develop in pockets of the axial cell. They leave the mother animal by tearing open the body wall and develop into new nematogens.
Alternatively, possibly if the population density is too high , a chemical factor induces the vermiform larvae to form another shape, called a rhombogen . Rhombogens and nematogens are anatomically very similar. The rhombogens develop a further stage in the pockets of their axial cells called the infusorigen . Infusorigenes do not leave their parent animal. They have hermaphroditic gonads through which they fertilize each other within the rhombogen. So-called infusoriform larvae , which are more complex than their mother animals, hatch from the fertilized eggs . At the rear end, these larvae have a dense bundle of cilia with which they can swim. The larvae are released from the host into the open water with the urine. They then look for a new host, the details of the infection being unknown. The first nematogens in the new host have three instead of one axial cells and are called stem nematogens.
ecology
The animals live in the excretory organs of cephalopods . They are mostly seen as their parasites . Since they promote the ammonia excretion of their host by acidifying their environment , they can also be regarded as endosymbionts .
The infection rate of cephalopods with rhombozoa is generally very high; in many populations almost all cephalopods are affected. As a rule, nematogens predominate in young animals and rhombogens in older animals, although it is not precisely known why. The Rhombozoa occur mostly in only one or in a few closely related cephalopod species, so they are host-specific. A single cephalopod species can, however, be infected by several rhomboid species. So are z. B. on the California coast 18 species of Rhombozoa are known to infect eight species of cephalopods.
Systematics
The Rhombozoa are an animal group with an uncertain phylogenetic position. These are secondary simplified tissue animals from the Bilateria . In the past they were combined with the Orthonectida , with which they have some similarities, to form the taxon " Mesozoa ", which however is not a natural group, ie not a " monophyletic taxon ". Today two hypotheses about their systematic position are widespread. According to the first, it is a very basic, primitive Bilateria that is not closely related to any other animal phyla. Most systematists today, however, assign the Rhombozoa to a family group, the Lophotrochozoa , which u. a. also includes the mollusks and the annelid worms. As there are hardly any morphological features due to the extremely simplified body structure, the assignment is based on similar, homologous DNA sequences in so-called molecular pedigrees.
The Rhombozoa are divided into two groups:
- Dicyemidae (approx. 75 species):
- Nematogens and rhombogens have a ciliated epidermis and the above-mentioned life cycle.
- Heterocyemidae (2 species):
- The epidermis of the nematogens and rhombogens is not eyelashed and there are further variations in structure and reproduction.
literature
- Wilfried Westheide & Reinhard Rieger (Hrsg.): Special Zoology - Part 1: Protozoa and invertebrates (2nd edition). Elsevier, Spektrum Akademischer Verlag, Munich 2007, ISBN 3-8274-1575-6
- Sol Felty Light & James T. Carlton: Dicyemida (Rhombozoa). in: The Light and Smith Manual. Intertidal Invertebrates from Central California to Oregon, Fourth Edition, University of California Press 2007. ISBN 9780520239395
- NB Petrov, VV Aleshin, AN Pegova, MV Ofitserov, GS Slyusarev (2010): New insight into the phylogeny of Mesozoa: Evidence from the 18S and 28S rRNA genes. Moscow University Biological Sciences Bulletin 65 (4): 167-169. doi : 10.3103 / S0096392510040127 .
- Takahito G. Suzuki, Kazutoyo Ogino, Kazuhiko Tsuneki, Hidetaka Furuya (2010): Phylogenetic Analysis of Dicyemid Mesozoans (Phylum Dicyemida) From Innexin Amino Acid Sequences: Dicyemids Are Not Related to Platyhelminthes. Journal of Parasitology 96 (3): 614-625. doi : 10.1645 / GE-2305.1