Class Rhabditophora
** Examples, Characteristics and Reproduction
Class Rhabditophora is of the phylum Platyhelminthes (flatworms) and includes most parasitic flatworms (including members of Trematoda, Monogenea, and Cestoda) as well as free-living flatworms that have been previously classified under the class Turbellaria.
It's a sister group of Catenulida and has the following characteristics:
- Duo-gland adhesive systems
- Rhabdites
- Multiciliate protonephridia
- Macrostomorpha with aflagellates and biflagellate
Overview/Characteristics
Ecology
Given that the clade contains both parasitic and free-living organisms; its members can be found in both aquatic and terrestrial environments. For instance, according to studies, over 200 free-living species (Rhabditophorans) have been identified in freshwaters ecosystems in North America. However, these species are more likely to be found in temperate and tropical zones.
Apart from freshwater, some species have been identified in sandy shores, brackish, and marine environments (particularly on the seafloor for those in marine environments). In these environments, free-living rhabditophorans live as predators and feed on such organisms as bacteria, protozoa, and algae.
* In their environment, free-living rhabditophorans feed on their prey through a proboscis (a mouth-like structure that is located at the end of the trunk-like pharynx). This structure normally protrudes from the mid-ventral part of the body during feeding.
Parasitic members can also be found in aquatic and terrestrial environments where they depend on a host for their survival. For instance, rhabditophorans in aquatic environments live as external parasites of fish and other aquatic invertebrates.
By attaching themselves to these organisms, some species not only obtain nutrients from the host but are also transported to the definitive hosts. In terrestrial environments, parasitic forms infest such animals as cattle as their intermediate host and others like cats, dogs and human beings as the final host. However, their eggs can be found in different habitats on land.
Reproduction
Class Rhabditophora have been shown to reproduce both sexually and asexually.
Asexual reproduction among organisms like Planarians is rare compared to sexual reproduction. In these organisms, asexual reproduction has been shown to occur once a month and mostly in the dark. During asexual reproduction, adult forms differentiate into units known as zooids that ultimately separate at the middle (transverse division).
This mode of reproduction is aided by the regenerative ability of flatworms that allow the newly divided forms to grow back to the normal adult size.
While some species reproduce asexually, sexual reproduction is the most common form of reproduction among class Rhabditophora. However, some of the species are capable of both sexual and asexual reproduction.
While some of the species are gonochoristic, and are therefore either individual male or female, the majority are hermaphroditic and thus have both male and female reproductive systems.
During this form of reproduction (sexual reproduction), individuals exchange gametes which allows one individual to fertilize the eggs of another. However, for some of the species, self-fertilization is possible and reproduction does not require an exchange of gametes.
Following gamete exchange, eggs are laid on substrates in their environment. Under favorable conditions, tiny flatworms emerge from the eggs but remain attached to the substrate where they develop further. For a majority of species, there is no larval stage and thus the organism does not have different developmental stages.
* Under unfavorable environmental conditions, class rhabditophora produce resistant eggs.
* Whereas species that reproduce sexually only live for several weeks or months, those that reproduce asexually exist for much longer.
Class Rhabditophora Morphology
Morphologically, class Rhabditophora are bilaterally symmetrical with a triploblastic organization. As such, they are among the simplest animals consisting of three tissue layers (ectoderm, mesoderm, and the endoderm). Depending on the organism, the mesoderm may be less well defined. Different parts of the body of these organisms originate from these layers.
For instance, the reproductive system, parts of the excretory system, muscles and the connective tissue all originate from the mesoderm. During early development, the ectoderm develops to form the outer covering of the organism while the endoderm gives rise to the digestive sack.
Muscle fibers of the organism are organized in such a manner that they rest along its body wall. Here, the muscles are normally bundled so as to produce several layers that include; a thin intermediate diagonal layer, inner longitudinal layer as well as an outer circular layer. The connective tissue, on the other hand, is located between the muscles and digestive tissues.
* While class Rhabditophora lack a circulatory system, their flattened body allows them to obtain oxygen through diffusion.
In such organisms as triclad, which are among the largest rhabditophorans, studies have identified a well defined digestive cavity that consists of a single anterior gut that branches into two (a series of diverticula emerging from the two branches act as lateral body tissues).
The nervous system in rhabditophorans is characterized by various patterns and a ganglionic brain. For species in marine environments, the nervous system consists of three pairs of nerve cords that are connected to each other by commissures that may exhibit a ladder-like appearance.
Cilia
To move from one point to another, organisms like Planarians use their ventral cilia. Here, cilia originating from the epithelial cells are used for gliding along surfaces in a type of movement commonly referred to as ciliary-gliding. This is particularly important for free-living rhabditophorans given that it allows them to not only traverse substratum but also hunt for prey.
In addition to gliding, studies have also shown these organisms to use muscle movement to negotiate obstacles. Here, it is thought that PC2 (pro-protein convertase 2) inhibition tends to influence muscular movement.
Rhabdites
One of the other defining characteristics of some in class Rhabditophora (e.g. Planarians) is the presence of specialized, rod-like secretory granules known as rhabdites. Located on their body surface, these granules are responsible for the production of a layer of viscous slime.
These structures are embedded in the tegument and duo-gland adhesive structures and play an important role in lubrication, adhesion as well as predator deterrence as suggested by some researchers.
By lubricating the surface of the organism, the slime not only allows it to adhere to given surfaces but also contributes to movement in general. It protects and prevents damage to the soft body of these organisms in their environment as they move from one point to another and glide over substrates.
Understanding phylum Platyhelminthes
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References
Claire G. Stevenson and Wendy Scott Beane. (2010). A Low Percent Ethanol Method for Immobilizing Planarians.
Gregory A. Lewbart. (2011). Invertebrate Medicine.
Matthew J. Hayes. (2017). Sulphated glycosaminoglycans support an assortment of planarian rhabdite structures.
Steven A. Ramm. (2017). Exploring the Sexual Diversity of Flatworms: Ecology, Evolution, and the Molecular Biology of Reproduction.
Links
https://louis.oercommons.org/courseware/module/740/student/?task=2
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