Are all animals coelomate
ONYCHOPHORA: PROPERTIES, NUTRITION, REPRODUCTION, SPECIES - BIOLOGY - 2021
The Onychophores are a strain of animals that are characterized by having an elongated body with a certain number of extensions on the sides that allow it to move properly on the substrate.
They are really ancient animals as the first fossils to be extracted from them date from the Cambrian in the Paleozoic Era. However, they were first described in the 19th century by the British naturalist Landsdown Guilding.
Sample of Onychophore. Source: Bruno Vellutini from São Paulo / São Sebastião, Brazil
These animals are mainly found in environments where moisture is prevalent. This is because they have very thin skin and cannot counteract the dehydration caused by harsh environmental conditions. Likewise, they live in places far from sunlight and only come out of them at night to hunt their prey.
Onychophores are animals that are part of the eukarya domain and, as such, consist of eukaryotic cells, in whose nucleus is the genetic material (DNA) that makes up the chromosomes.
They are multicellular and consist of several cell types, each of which specializes in a specific function.
In addition, onychophores are coelomated. This means that they have an internal cavity called a coelom that is of mesodermal origin. The koelom is important because it contains the animal's internal organs, although in these it only surrounds the gonads.
If an imaginary line is drawn along the animal's longitudinal axis, two exactly equal halves are obtained, which allows us to confirm that these animals have bilateral symmetry.
They are dioecious because the sexes are separated and also show sexual dimorphism. The females are usually larger than the males.
They reproduce mainly sexually with internal and external fertilization (depending on the species). They can be oviparous, viviparous, and ovoviviparous.
Onychophores have an elongated body that gives the impression of being flattened dorsally ventrally. Although most only measure up to 10 cm, specimens have been found that have exceeded this size and have reached more than 20 cm.
In general, its coloration is dark, showing colors that go from black to dark brown through green. There are also some that have slightly more vivid colors like orange.
They don't have articulated legs themselves, but they have some kind of limb that they can use for movement and locomotion. The number of these varies depending on the species.
The body is divided into two regions: head (anterior) and trunk (posterior). There isn't a very clear line between the two, so only someone who is very knowledgeable about these animals can point out the lines between them.
Morphology of an onychophore. Source: Lansdown Guilding
The most noticeable feature of the onychophores' head is a pair of forward-facing antennae. At the base of each antenna there is a sense organ that functions as an eye.
Below the antennae are additional attachments known as oral papillae. These are very important to the animal's feeding process as they are responsible for driving out the fluid that is paralyzing the prey.
On top of the head is also the mouth from which the jaws emerge, which are another pair of appendages here.
The back surface of the head is plagued by chemo-receptor papillae, which have a sensory function. They are particularly common on the antennae.
The most noticeable elements of the body are the extensions that come out of it and which many insist on loving legs, but which are not so. The correct name is Lobopods. The number of these varies depending on the species.
Each has 3 to 6 trailing pads that are in constant contact with the ground.
The body wall of onychophores consists of three layers. From the outermost to the innermost they are: the cuticle, which is made of chitin, thin and very flexible; the epidermis; and finally, several layers of smooth muscle tissue.
The taxonomic classification of onychophores is as follows:
Heterotrophs - carnivores
Onychophores are heterotrophic organisms, which means that they are unable to synthesize their own nutrients. Because of this, they have to feed on other living beings or on other substances.
With this in mind, it has been found that these animals are predatory carnivores, the diet of which is mainly represented by a wide variety of animals such as arthropods.
The size of the prey does not seem to be a limiting element on this diet, as they will eat small animals as well as animals that are slightly larger than themselves.
Thanks to the receivers located on their antennas, they can also perceive possible prey from a distance.
Once the prey is identified, the animal throws a kind of silk, the function of which is to immobilize it. It is important to note that the silk is initially in a liquid state. However, when it comes into contact with the environment, it goes through a solidification process and becomes a network that captures and immobilizes the prey.
It should also be mentioned that the animal can throw this substance at distances of up to 50 cm.
Once the prey has been immobilized, the onychophore approaches it and injects it with a substance in which certain digestive enzymes are dissolved. These have the function of processing and digesting the prey's tissues to facilitate the digestive process.
After the prey's tissue has been processed and turned into pulp, the onychophore ingests it. In the body, this nutrient juice travels from the oral cavity to the pharynx and later to the esophagus.
Then it goes to the intestines, where the nutrient absorption process takes place, and passes it on to the circulatory system to distribute it to the various cells.
Substances that are not used by the animal, either because they are not necessary or because it is unable to digest and ingest them, follow the digestive tract towards the end of the intestine. Eventually, they are released into the external environment in the form of feces through the anal opening.
Onychophores are dioecious animals, which means that the sexes are separate. There are female and male persons. In them you can see the two modes of reproduction that exist: asexual and sexual.
- Asexual reproduction
This type of reproduction does not include the union of male and female gametes. Because of this, the individuals created by this process are exactly the same, both genetically and physically, as their parents.
There are various asexual reproductive processes. In one of the species of onychophores in which only female individuals occur, the mode of asexual reproduction is parthenogenesis.
Parthenogenesis consists in the unfertilized egg cells that initiate a process of division and segmentation that is believed to be mediated by chemical or environmental factors, although it is not yet very well defined.
This process leads to the development of an adult female person. Of course, all copies obtained by this type of reproduction are exactly the same.
- Sexual reproduction
Sexual reproduction necessarily requires the interaction and fusion of male and female sex cells (gametes). These cells are represented by eggs and sperm.
The reproductive mechanism in these animals is very different and depends on each species. Despite the fact that fertilization is internal in all types of onychophores that reproduce sexually, the mechanism by which it occurs is different.
There are species in which copulation occurs and the male deposits the sperm directly in the female's body.
There are also species in which reproduction occurs through a structure known as a spermatophore. This consists of a large mass of tissue that contains the sperm. The male deposits the spermatophore on the surface of the female's body, which it then inserts into his genital pores.
Embryonic development in onychophores is also different as there are some species that are oviparous, others viviparous, and some ovoviviparous.
In the oviparous case, development occurs in an egg outside the mother's body. The segmentation in these eggs is superficial.
Most species are ovoviviparous, which means that they develop in eggs, but these remain in the female's body until the individual is fully developed.
Finally, there are also types of onychophores that are viviparous. In them, the embryo remains in the mother's body and feeds on it. The new individual is born fully formed.
It is one of the new types of onychophores recently discovered. The first formal description dates from 2013. It can be up to 6 cm long and has a series of characteristic hairs on the surface of the body.
They have scales on the ventral part of the body that have a specific arrangement that allows them to be differentiated from other onychophores. They are usually hidden in damp places and only come to the surface during the rainy season.
Eoperipatus totoro specimen
It has the great honor of being the first described onychophore, which appeared in 1826. It belongs to the Peripatidae family and is characterized by a rather dark, almost black color. It occurs mainly on the island of San Vicente in the Caribbean.
It belongs to the Peripatidae family and is found mainly on the Asian continent, particularly in the western part of Malaysia. It has the same elongated body as onychophores with two front antennas that it can use to spot potential prey or danger.
It belongs to the Peripatopsidae family and is endemic to northeast Australia. Like all members of this family, it has the most primitive properties of the onychophores.
- Barnes, R. (1977). Invertebrate zoology. New inter-American publisher.
- Brusca, RC & Brusca, GJ (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
- Curtis, H., Barnes, Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
- CP Hickman, LS Roberts, A. Larson, WC Ober & C. Garrison (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
- Morera, B. (2012). The onychophores, walking fossils. National University of Costa Rica
- Ríos, P. Onychophora. Excerpt from: https://academia.edu
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