What kind of animals is not found on our planet! Some amaze with their size, some surprise with their habits and lifestyle, others are distinguished by their incredible coloring.
But the most striking in terms of body structure are still sea and ocean inhabitants. Their body shape can be very unusual, as it has a special symmetry that is uncharacteristic of land animals. This is ray symmetry.
Types of body symmetry in animals
All animals can be divided into four groups according to the types of body symmetry:
- Animals with bilateral symmetry (bilaterally symmetrical). This group includes most species of terrestrial animals and a significant part of marine ones. The main feature is the arrangement of the organs of the body symmetrically with respect to one plane drawn through it. For example, the left and right side of the body, back and front.
- Radial symmetry of the body (radial symmetry). Characteristic for animals of the sea and ocean depths. The main feature is the structure of the body in such a way that several imaginary lines can be drawn through its central axis,relative to which body parts will be located symmetrically. For example, starfish rays.
- Animals with asymmetrical body shape. When symmetry is not characteristic at all, the shape is constantly changing depending on environmental conditions or on the movement of the animal. A typical example is the common amoeba.
- Lack of symmetry completely. These organisms include sponges. They lead an attached lifestyle, can grow on the substrate to different volumes and do not have a certain symmetry in the structure of the body at all.
Each listed group of organisms derives a certain benefit from its structure. For example, bilateral animals can freely move straight, turning to the sides. Animals with radial symmetry are able to catch prey from different angles. It is convenient for asymmetric organisms to move around and adapt to environmental conditions.
Ray symmetry: what is it
The main distinguishing feature of animals with radial symmetry is their unusual body shape. They are usually domed, cylindrical, or star or ball shaped.
Many axes can be drawn through the body of such organisms, with respect to each of them there are two perfectly symmetrical halves. This adaptation gives them the opportunity to have a number of advantages:
- They move freely in any direction, controlling all sides around them.
- Hunting gets bigger as prey is felt all aroundbody.
- The unusual shape of the body allows you to adapt to the surrounding landscape, blend into it and become invisible.
Beam symmetry of the body is one of the main adaptations for certain classes of animals of the oceanic biocenosis.
Characteristic of the radial symmetry of the body
The history of the emergence of such an adaptation as the radial symmetry of the body goes back to the ancestors of animals such as the Coelenterates. It was they who led a completely sedentary, motionless lifestyle and were attached to the substrate. They benefited from such symmetry, and they gave it a start.
The fact that now many actively swimming animals still have ray symmetry indicates its non-reduction in the course of evolution. However, this feature no longer fulfills its intended purpose.
Meaning of ray symmetry
Its main purpose in ancestral forms, as well as in modern, leading an attached lifestyle, is to provide protection from attacks by predators and get food.
After all, animals with ray symmetry were not able to protect themselves, having run away from a predator, they could not hide. Therefore, the only option for protection was to feel the approach of danger from any side of the body and respond in time with protective mechanisms.
Besides, earning your living when you lead a sedentary lifestyle is quite difficult. And the radial symmetry allows you to catch the smallest food sources around the entire body and quickly respond to them.
SoThus, the ray symmetry of the body provides extremely important self-defense and subsistence mechanisms for animals with it.
Animal examples
There are many examples of animals with radial symmetry. Their huge species and numerical diversity adorns the sea and ocean bottoms and water column, allows a person to admire the intricacy of nature and the beauty of the underwater world.
Which animals have ray symmetry? For example, such as:
- sea urchins;
- jellyfish;
- holothurians;
- ofiura;
- serpenttails;
- hydras;
- starfish;
- comb jelly;
- fixed polyps;
- some types of sponges.
These are the most common examples of ray symmetry of the body in animals. There are other animals, little studied, and perhaps not yet discovered at all, which are characterized by such a feature of the physique.
Celiac
This type of animal includes three main classes, a common feature of the representatives of which is that they are all animals with ray symmetry. In life cycles, either the stage of a free-floating jellyfish or the stage of a polyp attached to the substrate predominates. The hole is one, performs the function of oral, anal and genital. They use poisonous stinging cells for protection.
- Hydroids. Main representatives: hydras, hydrants. They lead an attached lifestyle, have, like all coelenterates, two layers in the body structure:ectoderm and endoderm. The middle layer is a gelatinous substance of a watery composition - mesoglea. The body shape is most often goblet. The main part of life passes in the polyp stage.
- Jellyfish (scyphoid). The main representatives are all types of jellyfish. The body shape is unusual, in the form of a bell or a dome. They are also two-layered animals that have ray symmetry. The bulk of life is spent in the free-moving jellyfish stage.
- Corals (polyps). Main representatives: sea anemones, corals. The main feature is the colonial way of life. Many corals form entire reefs from their colonies. Single forms are also found, these are different types of sea anemones. The medusa stage is not typical for these animals at all, only the polyp stage.
In total, there are approximately 9,000 species of representatives of this type of animal.
Echinoderms
What other animals have ray symmetry? Of course, everyone knows and very beautiful, unusual and bright echinoderms. This type has about 7 thousand species of these amazing representatives of marine fauna. There are five main classes:
- Holothurians - resemble worms, but still have radial symmetry. Brightly colored, move reluctantly on the seabed.
- British brittle stars - reminiscent of starfish, but are characterized by higher mobility and poor coloration - white, milky and beige colors.
- Sea urchins - may or may not have a regular, acicular external skeleton. Body shape is almost alwaysclose to spherical.
- Sea stars are five, eight or twelve-rayed animals with pronounced radial symmetry. Very beautifully painted, lead a sedentary lifestyle, crawl along the bottom.
- Crino lilies are sedentary beautiful animals, they have the shape of a radial flower. They can separate from the substrate and move to more food-rich places.
Lifestyle can be both mobile and attached (sea lilies). The body is two-layered, the mouth opening performs the function of anal and genital. The outer skeleton is quite strong, calcareous, beautifully decorated with colored patterns.
The larvae of these animals have a bilateral symmetry of the body, and only adults grow rays to radiality.
Comb jelly
Most often small animals (up to 20 cm), which have an absolutely white, translucent body, decorated with rows of combs. This type of animal is considered one of the most ancient. Ctenophores are predators, they eat crustaceans, small fish and even each other. They breed very intensively.
The third germ layer appears in the structure of the body. The mouth opening on the upper body, lead a free-floating lifestyle. The most common types are:
- beroe;
- platyctenides;
- gastrodes;
- venus belt;
- bolinopsis;
- tjalfiella.
Their radial symmetry, as well as the radial symmetry of some coelenterates, is weakly expressed. The shape of the body resembles a bag or an oval.
Summary
Thus, the ray symmetry of the body is the prerogative of aquatic animals that lead a sedentary or attached lifestyle and give their owners a number of advantages in hunting prey and evading predators.
