Definition, Characteristics, Phylogeny, Vs Rhabditophora
Definition: What is Catenulida?
Catenulida is an Order of the phylum Platyhelminthes consisting of turbellarian worms. Currently, about 100 species of the group have been identified with a majority existing as free-living organisms. While they vary in size, members of the Order Catenulida are generally small in size and can be found in a variety of habitats (moist terrestrial environments, ponds, mires, etc).
Ranked under the Phylum Platyhelminthes, catenulids also share a number of characteristics with other flatworms including the general morphology and means of reproduction.
* Catenulida was previously ranked as a Subphylum.
Some of the organisms that belong to the Order Catenulida include:
· Phylum: Platyhelminthes - Acoelomate flatworms characterized by an unsegmented flattened soft body
· Class: Turbellaria - A sub-division of the phylum Platyhelminthes consisting of mostly free-living flatworms that are widely distributed in nature
· Order: Catenulida - A group consisting of small, free-living flatworms that are commonly found in aquatic environments and moist environments
* The order (Catenulida) is made up of the following families:
Characteristics of the Order Catenulida
With the majority of species in the group being free-living organisms, they can be found in a variety of habitats where they feed on organic compounds/matter in their surrounding.
According to studies, the majority of these organisms (about 80) can be found in freshwater environments (in inland waters). However, other species, such as members of the family Retronectidae (with the exception of Myoretronectes paranaensis) are commonly found in marine environments where they live freely.
Other species have been found in such habitats as ponds, streams, mires, and moist moss. Given that they are capable of movement, these environments not only allow them to move about freely in search of food, but also in search of favorable conditions in which they can reproduce.
* In these environments, competition between some of the species is largely influenced by such factors as temperature and availability of food.
In their respective habitats, catenulids feed on such food sources as diatoms, algae, detritus, protozoans and other microscopic organisms.
General Biology/Morphological Characteristics
As mentioned, catenulids are relatively small worms measuring between 0.2 and 2.0mm in length. While differences in length are common between different species, differences have also been identified within species (This has been attributed to asexual reproduction of these organisms which tends to result in the formation of a chain of zooids). In reflected light, they appear white in color but appear translucent when viewed using transmitted light.
As is the case with all Platyhelminthes, the body surface of these organisms is covered by a gel-like layer known as a glycocalix/glycocalyx consisting of glycoprotein and carbohydrates. Together with a number of other substances, the glycocalyx layer is secreted by various glands located on the epidermal surface of the organism.
Based on molecular studies, cell bodies of these glands have been shown to originate from the parenchyma. These secretions are particularly important for these organisms in that they not only act as an additional shield covering the body surface but also contribute to locomotion by allowing catenulids to slide on a substrate.
While some of the species have been shown to have a basal matrix, it's not continuous between the underlying muscle and epidermal cells of these organisms. For this reason, they lack a uniform basal membrane compared to Rhabdocoela.
Like all Turbellaria, however, members of this group are characterized by the distribution of cilia on their body surface. As compared to Rhabditophora, which contain about six cilia per cell, there are about 3 cilia per cell in Catenulida (number of cilia is significantly greater at the pharynx).
Here, cilia are anchored to two roots (positioned opposite to each other) on the surface membrane of the organism. By reversing the beat of these cilia, catenulids are able to swim from one location to another in their environment.
* Catelunida cilia are of the axonemal pattern 9+2. However, compared to the other flatworms, the cilia of these organisms exhibit a progressive unilateral stepwise reduction of the double tubules forming single filaments towards the distal end.
Apart from cilia, catenulids have also been shown to poses rod-shaped rhabdites on their surface. Depending on the organism, they may serve protective or secretory functions.
Rhabdite-like structures produced by glandular cells in the parenchyma have also been identified on the surface of these organisms. Although the role of these structures is not fully understood, they are suspected to play an important role in capturing prey, evading predators, and cocoon formation among a few other functions.
For Catenulids, like many other turbellarians, the parenchyma acts as the connective tissue. As such, it occupies all the free space within the body of the organisms.
Compared to the other groups, however, the amount of extracellular matrix in the parenchyma of Catenulids is very little or absent. As a result, it's primarily composed of parenchymal cells.
As connective tissue, some of the main functions of this layer include organization of the epithelial layers, housing neoblasts that are involved in regeneration as well as nutrient storage.
As members of the phylum Platyhelminthes, catenulids have a central neural system consisting of a cerebral ganglion, orthogonal system as well as peripheral system.
When viewed under the microscope, the cerebral ganglion appears lobed in shape. The nerve cords, which are short in length, on the other hand, extend from the lobes to the pharynx as well as the posterior and anterior parts of the peripheral plexus.
In members of the Order Catenulida, the mouth opening (the oral pore) is almost completely frontal. This allows the organisms to swallow food material as they swim around in their environment.
Like some of the other groups in the phylum, the intestine of these organisms is ciliated, a feature that increases the general surface area of the intestine. The pharynx simplex, which has evolved in catenulids, is the result of an invaginated ectodermal and is also ciliated.
There are different types of pharyngeal glands with different arrangements (e.g. type a glands which are small and rounded and type b glands that are elongated and club-shaped). While some of the species lack this structure, others either have one or several types of these glands.
With regards to arrangement, the glands can be seen opening onto the junction between the mouth and pharynx in some species of uniformly distributed in others. The pharynx also consists of muscle and glands that make it possible for the organism to swallow whole prey. While they have a mouth part, they lack an anus. For this reason, waste products are removed through the oral pore/mouth opening.
Some of the other morphological characteristics associated with Catenulids include:
· Tend to form chains consisting of several individuals (these chains contain an average of 9 individuals)
· Both ends of the organisms are normally pointed
· Species that lack laminar rhabdites (e.g. Stenostomiade) have compact rhabdites known as rhabdoids (false rhabdites)
· A majority of the species have ciliated pits which are basically paired depressions located at the antero-lateral body wall of the organisms. When viewed under the microscope, they appear as rounded/elongated structures that are densely ciliated.
· For freshwater species, osmoregulation is through the protonephridial system (The protonephridia is made up of terminal flame cells). This system allows the organisms to filter out excretion products and excrete them through the protonephridial pores which are located at the pharynx pouch.
While Catenulids are capable of sexual and asexual reproduction, sexually mature organisms are rare. Regardless, male and female gonads have been identified in different species. While female gonads and ovaries have been identified in these organisms, studies have also shown that they lack female genital pore. For this reason, the body wall has to break in order for the eggs to be released.
Given that the free-living organisms in the group are hermaphrodites, sexual reproduction may occur through autogamy (self-fertilization). However, this can also occur through exogamy. So far, this mode of reproduction has only been well studied among members of the group Retronectidae.
Following fertilization, the eggs are laid on a substrate (in cocoons) where they hatch to produce larval forms. Over time, these forms mature into adults forms capable of reproduction.
Asexual reproduction, which is more common, involves the production of new worms from the posterior end of the organism. Here, a new worm grows from the posterior of the parent to form a chain of zooids. These zooids then develop to form separate organisms that can then detach from the parent zooid.
For the most part, then, a monophasic life cycle is common for members of this group given that new forms undergo direct development that results in separate independent organisms.
Historically, Catenulida, Rhabditophora, and Acoelomorpha were grouped as the three main clades under the phylum platyhelminthes. Being sister groups, the two share a number of characteristics that are common of all platyhelminthes.
Like Catenulida, Rhabditophorans are nonsegmented acolomates with a multiciliated epidermis. However, there are also a number of differences between the two.
Whereas Rhabditophorans contain six cilia per cell on their epidermis, Catenulids only contain about 3 per cell. With regard to cilia, differences are also evident in their arrangement patterns. For Rhabditophorans, a continuous narrowing of the axoneme is exhibited while a progressive unilateral stepwise reduction of the double tubules into single filaments is observed in Catenulids.
Similarities between the two groups have also been identified in their central neural system. Apart from such parts as the central nervous system and nerve cords, etc, Rhabditophorans have also been shown to contain a neuropil found in Catenulids. One of the main differences is that the cerebral ganglion of Rhabditophorans is surrounded by a capsule that is absent in Catenulids.
Like Catenulids, Rhabditophorans also have a mouth opening/oral pore. However, this is posterior ventral or medial-ventral with the exception of very few species. This is different from the oral pore of Catenulids which is almost frontal allowing these organisms to swallow various food materials in their surrounding as they swim about. With regard to reproduction, both groups are capable of sexual and asexual reproduction. Differences, however, are evident in their reproductive systems.
As mentioned, Catenulids lack a female genital pore. For this reason, the body wall has to break in order for eggs to be released during sexual reproduction (a process that causes the organism to die). Rhabditophorans, on the other hand, have separate female and male genital pores with the female genital pore being anteriorly located to the male genital pore.
While the two groups share many general characteristics (morphology, reproduction, and digestion, etc), they have various essential differences that have helped shed some light into their individual evolutionary pathways.
Phylogeny of Catenulida and Support for Platyhelminthes
Through bilaterian phylogeny studies based on morphological characteristics, many studies have shown Catenulida, among several other groups (e.g. Nemertodermatida and Rhabditophora) to be basal bilaterian groups. Through these morphological studies, however, other groups (Acoela,Nemertodermatida and Rhabditophora) were found to be the most basal bilaterian groups followed by Catenulida.
Apart from morphological studies, many others focusing on rDNA have also been conducted to determine the phylogenetic position of this group.
Through these, Catenulida was not only shown to be the sister group of Rhabditophora, but also a group belonging to the clade Platyhelminthes.
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