Rhombozoa . The rhombozoans or dicyemids are an order of invertebrate animals , consisting of small parasites of fish and molluscs . They live in mucous membranes and elsewhere with a virtually anaerobic environment. There are approximately 75 species.
Summary
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- 1 Son Rombozoos
- 1Orden Dicyemida
- 1.1 Development of the vermiform
- 1.2 Development of the infusiform
- 2ORDEN heterocYEMIDA
- 2.1 Knowledge gender
- 2.2género microcyema
- 2 Sources
- 1Orden Dicyemida
Son Rombozoos
In some classification it can be found together with the Phylum Orthonectidae grouped in the Phylum Mesozoos . The Rhombozoa are animals that live inside the excretory ducts of the Cephalopods. Despite their parasitic life, the damage they cause to the host is reduced to the fact that, when the density of rhombozoans is very high, the excretory ducts become clogged (at normal density there is no apparent damage to the cephalopod). Two orders are distinguished: Dicyemida and Heterocyemida .
Order Dicyemida
During its life cycle, a state called “vermiform individual” or “nematogen” appears. This phase is the one found inside the cephalopod. The body of a vermiform is elongated, provided with a layer of external ciliated cells and whose number is fixed for each species (a number that is around 20 cells). At the anterior end of the body, 8-9 cells differentiate that constitute a polar capsule that serves to attach to the walls of the excretory ducts. During juvenile stages, the animal swims through the urine of the cephalopod and, when it becomes an adult, it will become attached to the excretory duct. The cells of the outer layer (ciliated) serve for locomotion in juvenile stages, while in the adult state they serve to maintain a current of liquid that flows around the body of the animal.Axial cell .
Within this axial cell we will find another type of cells, with a number between one and more than one hundred, which is known as Axoblasts . During the life cycle there is a stage of Asexual reproduction and another stage of Sexual reproduction. When the conditions of the environment are favorable for the life of the Rhombozoan (adequate density of them) asexual reproduction occurs. For the phenomenon, the axoblasts intervene, which form a new vermiform individual that crosses the maternal body and passes to the excretory ducts.
At first it is free-living and then settles to give rise to an adult individual. Under adverse conditions (alteration of the ducts, high density of individuals…) an Infusoriform individual is formed from the axoblasts.rounded appearance, with a layer of ciliated cells on the outside except at its anterior end, which has two apical cells lacking cilia. Four central cells are located inside the animal. The individual leaves the cephalopod and will go into the water. As we already mentioned, axoblasts are a type of cell that are located inside the axial cell. These cells are responsible for the two types of cells: vermiform and infusoriform.
development of the vermiform
From the axoblast there is a cell division that originates two daughter cells with a very different size: macromere and micromere . The micromer will successively divide forming new micromers . Said new micromers tend to envelop the macromer . Once the macromere is enveloped by the new micromeres, it will divide to give rise to a new macromere and another micromere .. The macromere phagocytizes the micromere, which will remain inside it. In this stage, the outer cells (micromeres) will give rise to somatic (outer) cells of the vermiform animal. Internally, the macromere is located, which will constitute the axial cell. Finally, the micromere that is inside the axial cell will constitute the first axoblast of the new vermiform individual. From the first axoblast the first vermiform develops, since the external cells divide until reaching the typical number of each species. The axial cell will develop along with the individual. From the first axoblast, by successive divisions, new axoblasts will originate . Once the individual has developed, it crosses the maternal wall and passes into the urinary ducts.
development of the infusiform
We start from an axoblastwhich will split giving a macromer and a micromer. Once formed, the micromere degenerates and disappears. For its part, the macromere divides again and originates a new macromere and a micromere. Both the macromere and the micromere split. The micromer gives rise to two micromers, and from the macromer one macromer and one micromer are formed. The macromere phagocytizes the micromere and, at the same time, the two remaining micromeres will successively divide tending to envelop the macromere. As the micromeres divide, enveloping the macromere, they will develop into female cells. For their part, the micromeres previously phagocytosed by the macromere divide inside it to form male gametic cells. We find a stage that some authors call ” hermaphroditic gonad” or Infusorigen . The male internal cells fertilize the female cells and from the egg the infusoriform larva develops through several divisions. We could call this stage of the cycle the sexual phase of life (the asexual phase would be the one that corresponds to the development of the vermiform individual).
ORDER HETEROCYEMIDA
This order is made up of two species that belong to the genera Conocyema and Microcyema.
Nenero conocyema
The life cycle is similar to what has been explained so far, with the formation of vermiform and infusoriform individuals . In vermiform individuals we find that the somatic cells lack cilia and at the polar end it is made up of only four highly developed cells. Internally we can find the axial cell and the axoblasts . The life cycle is similar to that of the other order studied, with vermiform individuals in favorable conditions, and infusoriform individuals in adverse conditions. When the infusoriforms are going to form, a series of vermiform individuals intervene in which the four cells that we mentioned above disappear.
genus microcyema
There are vermiform and infusoriform individuals. Vermiforms have the outer layer formed by a syncytium. Inside it is the axial cell and the axoblasts. During the life cycle there are two processes for the formation of vermiform individuals in which the axoblasts always intervene :
- It goes through a juvenile stage in which differentiated cells appear in the outer layer instead of remaining as a syncytium. Later, these cells will disappear, creating a stage in which the individual has an ameboid appearance and in which the formation of the syncytium is observed externally and inside which is the axial cell. From the amoeboid the new adult vermiform will develop.
- It goes through the differentiation of a larval stage known as ” Wagener’s larva “, in which it has an axial cell inside and is surrounded by several cells provided with cilia. From here, by metamorphosis, the new adult individual will be reached. During the formation of the infusoriform it is very similar to the previous one and there is no morphological differentiation as it happened before.