Secondary animals: classification

A feature of second-term animals is that during the development of the embryo the formation of the anus occurs on the place of formation of the primary mouth, and the mouth subsequently appears in a completely different place. In other words, it can be said that the embryo has a mouth opening from one end, and an adult specimen in the opposite place. Secondary animals belong to the subsection of the kingdom, which includes echinoderms, chordates and semi-chordates. They are referred to the so-called bilaterally symmetrical living organisms.

Features of bilaterally symmetrical animals

The main feature of such animals is that the left and right sides of their bodies are clear mirror images of each other. How can this be understood? Just need to imagine a plane that divides the animal's body in half. In this case, both these parts will completely coincide. In some sources one can come across the notion of "bilaterally symmetrical" animals.

This feature completely distinguishes this species from other fauna and human species, in which the body is conditionally symmetrical. This means that not all organs are placed in the same plane. And for bilaterally symmetric other planes there. This feature has positive aspects. Such animals move very easily along straight lines and turn. These include primary and secondary animals.

The difference between primary and secondary

Representatives of these species, although similar, but the differences are still there. As it was said, first-term and second-term animals are bilaterally symmetrical. These names come from how their mouth opening develops during embryonic development. In primary moles, a blastospore transition occurs (an opening in the primary intestine) in the formation of the oral cavity. And in the second-term in this place the anus is formed. In this case, the mouth opening is formed in a new way at the anterior end of the embryo. There are also examples when the blastopore completely closes, and the mouth and anus appear again.

And another important difference is the development of the primary brain. The primary brain develops the brain of an adult animal. In the secondary, its reduction takes place, and the new one forms again elsewhere. Secondary animals are also called secondary cerebral animals.

Classification of secondary animals

Above we considered who are the second-term animals, examples and features of their development. Now it's time to find out who is related to this sub-kingdom. This includes the following types:

- chordates;
- bristle-jaw;
- echinoderms.

Now let's consider in detail which animals belong to the second-term animals. Among the chordates are lancelets, lamprey, fish, amphibians, reptiles, birds and mammals. Bristle-mandibular are marine animals, the most famous of which is the sea arrow. A very unusual type of echinoderms includes sea stars, sea urchins, holothurians, sea lilies. All these representatives of the fauna are united by the fact that they are secondary animals. In addition to the features of the formation of the oral cavity, these creatures also have differences in the development of other organs and systems.

Specific features of development of chord embryo

Chordates are second-term animals, which differ in appearance, appearance and conditions of life. Representatives of this type can be found everywhere. They live on land, in water, in the soil, and in the air. Distributed throughout the globe. The number is about forty thousand.

They all have the presence of an axial skeleton, which at the stage of development of the embryo is represented as a dorsal string (chorda). In adult individuals, in an unchanged form, it remains only in the lower representatives of the type. In all the others, it passes into the formation of the spine, which goes from solid to segmented.

The method of crushing a fertilized egg is also a distinguishing feature between these two sub-kingdoms: the spiral in the primary and the radial in the second-grained.

The nervous system is a hollow tube, the front part of which later becomes the brain. From his inner cavity his ventricles form.

In the anterior section of the digestive tube there are two rows of holes, through which there is a communication with the external environment. These are the so-called visceral gaps. The lowest representatives of chordates in this place have gills. All the rest are only germinal rudiments, which subsequently do not function.

Some sources refer to the second-term so-called semi-chords. These are vermiform benthic animals. Characterized by the presence of a notochord (chord-like organ) and paired gill slits. In embryonic development they resemble chordates, but the structure of the body is completely different. The body is represented by three divisions: a proboscis, a collar and a trunk.

Bristle-teeth

These animals are sea predators with the ability to move quickly. Outwardly they resemble an arrow that is sharpened from the front end, and on the back has feathering. This is the very setae with which the animal captures food. The body consists of the head, trunk and tail. There are paired lateral and one caudal fins.

The belonging of these animals to the secondary ones consists in the embryonic development of the oral cavity and in the fact that the egg is crushed radially. In all other respects there are a number of differences. These animals have no blood, respiratory and excretory systems. There are also no genital ducts. The nervous system is an oclo-pharyngeal ring.

Features of echinoderms

A characteristic feature of representatives of this type is the presence of an ambulacral system. These are fluid filled cavities, through which the animal moves, breathes, touches and produces excretory processes.

The intestine is a long tube or bag. The circulatory system is represented by annular and radial vessels. Decay products are released through small pores in the walls of the body. Weakly developed sensory organs and nervous system. But regenerative abilities are well developed. In case of danger, these animals can discard individual parts of the body that are restored in two weeks. Due to this feature , the starfish can even multiply by dividing in half. Through time, the second half is fully restored.

Results

From all of the above, you can find out which animals belong to the second-term, the features of their development and the representatives of this sub-kingdom. Obviously, representatives of this species are very interesting. Their research is still ongoing.