The polychaetes They are a class of animals belonging to the phylum annelida. They are characterized by being segmented and have appendages called parapods, which are organized in pairs, in each segment.
This class was first described in 1850 and is made up of a wide variety of organisms, many of which have the ability to move freely. On the other hand, others are sessile.
These animals are found in marine habitats. According to specialists, they are capable of resisting wide ranges of salinity, some being benthic. Polychaetes constitute a very interesting group of living beings, which still continues to be the object of the most diverse investigations.
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The taxonomic classification of polychaetes is as follows:
- Domain: Eukarya.
- Animalia Kingdom.
- Phylum: Annelida.
- Class: Polychaeta.
Polychaetes are multicellular eukaryotic organisms. This means that their genetic material is enclosed in the cell nucleus. Likewise, it is made up of varied cells that have specialized in various functions..
Likewise, they present bilateral symmetry, that is, if a line is drawn along the longitudinal plane, two exactly equal halves will be obtained..
These animals are heterotrophic organisms, since they are not capable of synthesizing their own nutrients. They are often carnivorous and in some cases feed on sediments.
It is a fairly large group, which includes organisms with extraordinary mobility, as well as others that are sessile and remain fixed on the seabed..
Although they are quite simple animals, the organic systems that make them up have a certain level of complexity compared to other members of the phylum annelida..
Polychaetes, like all organisms belonging to the phylum annelida, have a clearly segmented body. Each segment is known as a metamer. They have a variable size, ranging from a few millimeters to 3 meters. However, the average common size of these animals is only 10cm..
This group of animals is characterized by having appendages that are detached from each metamer. These appendages are known by the name of parapods or podiums. From an evolutionary point of view, parapods are considered the first paired locomotive appendages. Its function is related to the movement of the animal.
Each parapod is in turn divided into two portions, an upper one known as the notopod and a lower one called the neuropodium. It is important to note that in the case of presenting the two mentioned branches, the parapod is called birrámeo, while if it only presents a single branch, it is known as unirrámeo.
Likewise, the polychaetes present extensions at the level of the parapods that are called silks. These are similar to bristles and are made up of a polysaccharide called chitin..
The body is divided into three zones or areas: the head, also called the prostomium, the trunk or metastomy, and the pygidium..
It is made up of two parts: the peristomium, which is the segment in which the mouth opens, and the prostomium, which is a kind of preoral lobe that can include some sensory-type structures such as antennae, cirrus, and eyes, among others..
It is important to mention that despite the fact that in some species the peristomium and prostomium present an evident separation, in most species this is not appreciated..
In most cases, the mouth is surrounded by jaws made of chitin, which contribute to the capture and redirection of food towards the oral cavity..
This is segmented, being able to present two types of segmentation: homonomous or heteronomous. In the first case, the metamers that make up the trunk are the same, with the same internal structures.
In the case of heteronomous segmentation, the metamers present certain differences, which causes different regions to be established in the trunk.
It should be noted that each metamer presents the podiums, each one with its respective silks.
It is the final segment of the animal. It contains an orifice corresponding to the anus, which is the one through which the waste substances produced by digestion are released..
Seen a section of the wall of a polychaete under the light microscope, the presence of several layers becomes evident:
- Cuticle: it is the outermost layer of the animal. It is very thin and transparent. Its purpose is to protect you against any predator or threat to your integrity.
- Epithelium: made up of epithelial and glandular cells. Likewise there are also sensory cells present.
- Basement membrane: thin layer that separates the epithelium from the underlying muscle layers.
- Muscle layers: first a circular muscle layer and then a longitudinal muscle layer. The latter forms longitudinal bundles that can be one or two dorsal and two ventral. These muscles contribute to the movement of the parapods.
- Somatopleura and splagnopleura: they are the innermost layers of the animal's wall. Both make up the septa and mesenteries that hold the digestive tract in place..
The digestive tract is made up of areas specialized in specific functions.
In general, the digestive system of polychaetes is made up of: mouth, pharynx, esophagus, stomach, intestine and rectum. Now, depending on the species and its food preferences, these structures can undergo certain modifications.
Predatory species have a pharynx that can evacuate, forming a proboscis that contributes significantly to the prey capture process. Likewise, in some specimens the esophagus has a smooth surface and in others its surface is ciliated..
In the stomach a series of digestive enzymes are synthesized that have the function of the degradation of ingested food.
The intestine is the organ specialized in the absorption of nutrients. It presents some species of sacs, called intestinal cecum, which is where the absorption of nutrients takes place. Thanks to the existence of blinds, the absorption surface is notably increased.
In the case of polychaetes who have sedentary lifestyles, the digestive tract is much simpler, since they ingest small particles of food, much easier to digest..
As occurs in all members of the phylum annelida, the excretory system of polychaetes is made up of nephridiums, which are located in pairs in each metamer of the animal..
Nephridia have two orifices, one that opens towards the coelom and is known as a nephrostoma; and another that opens outwards and is called a nephridiopore.
In many types of animals, including polychaetes, nephridia fulfill double functions: release of waste substances (excretory function) and release of gametes to the outside environment. Now, in polychaetes, nephridia can be of two types: nephromixus and myxonephros..
Nephromixes present a clear differentiation between the excretory portion of the nephridium and the coelomoduct, which is where the gametes are released. In the case of myxonephros, there is no obvious separation between the excretory area and the coelomoduct, since both are fused.
The nervous system of polychaetes is similar to that of other members of the phylum annelida. It is made up of a cerebroid ganglion that is located above the pharynx. It also has a periosophageal ring and two ventral nerve chains.
In the case of the more developed polychaete specimens, the brain has three lobes: anterior, middle and posterior. The anterior lobe innervates the palps and the tube, the posterior lobe innervates the nuchal organs and the dorsal integument of the head, and finally, the middle lobe innervates the eyes and antennae..
Similarly, the sensory organs are widely developed in polychaetes. They present the following:
- Photoreceptors. They can be of four types: simple eyes, pit eyes, chamber eyes and compound eyes. They perceive light-type stimuli.
- Statocysts They have to do with maintaining balance in polychaetes.
- Nuchal organs. They have a ciliated appearance and are located in the nuchal region of the animal. They have a chemoreceptor function, contributing greatly to the uptake of prey and food in general..
Most of the species included in the group of polychaetes are dioecious, that is, there are female individuals and male individuals.
The gonads, which is where gametes are produced, are found in the so-called genital segments (more evolved polychaetes) or in all segments (more primitive polychaetes).
Likewise, it is important to note that the level of specialization and differentiation in the gonads is not much, since they are made up of clusters of immature gametes that are finally released to the coelom, where they carry out and conclude their maturation process.
Polychaetes have a closed circulatory system. This implies that there are no vascular lagoons. Blood circulates mainly through two vessels: one dorsal and one ventral. In each one the blood circulates in opposite directions.
It is worth mentioning that both vessels are not isolated from each other, but are connected by transverse blood vessels in each metamer.
Likewise, hemoglobin and some pigments such as erythroquorin and hemeritria, among others, circulate in the blood. These provide characteristic colors to the blood of each animal.
The group of polychaetes is very diverse, so their food preferences do not follow a uniform pattern. There are predatory, omnivorous, herbivorous, scavenger, sedimentivorous or selective organisms, as well as filter feeders..
Taking this into account, there are polychaetes that hunt their prey through the jaws of their pharynx. Others feed on algae, small invertebrates, debris, and sediment.
Once the food is ingested, it passes through the esophagus into the stomach. There, thanks to the secretion of certain digestive enzymes, it is processed and converted into simpler molecules to be assimilated and absorbed at the level of the intestine, especially in the blind that comprise it..
Finally, digestion wastes are released through the anus..
Both types of reproduction are observed in polychaetes: asexual and sexual, the latter being the one most frequently observed.
This type of reproduction does not imply the fusion of gametes, nor the participation of some other individual. The most common forms of asexual reproduction that have been observed in the polychaete group are budding, excision, and stolonization..
Budding consists of the formation of gems somewhere in the animal, from which new individuals begin to develop.
In excision a fragmentation of the animal occurs. The cells that make up this fragment undergo a process of differentiation and specialization to give rise to a new individual..
Finally, in stolonization, a chain is formed in the final part of the animal that separates from the mother. From that chain a new individual develops.
Sexual reproduction involves the fusion of gametes, female and male. A very interesting phenomenon occurs in polychaetes, which is called an epitochy. This consists in that some polychaetes undergo certain morphological changes during their mating season..
The process is as follows: when the reproductive period arrives, the posterior end of the polychaetes' body is modified, and the parapods and quetas flatten, turning into swimming plates. Those modified parts are called epitochs..
These move towards the surface and form swarms with those of other specimens. In this way, the chances of cross-fertilization are increased. Female fragments secrete chemicals (pheromones) to attract males.
Fertilization is external and the development of the resulting individuals is indirect. From fertilization, a trochophore-type larva is formed that is planktonic. This larva undergoes a series of changes and transformations until it generates an adult individual.
The class of polychaetes mainly includes two subclasses: Aciculata and Sedentaria.
This subclass includes specimens that have a wide mobility and ability to move. This is due to the fact that they have modified needle-shaped ketas (needles), associated with an efficient muscular system, which allows the animal to move easily and quickly..
As its name allows to infer, the organisms of this subclass lead a sedentary lifestyle, since they are fixed in the substrate. The quetas are evidently modified. This subclass includes tubicultural organisms, which live in tubes made of chitin; and the excavators, who live buried in mud or sand.
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