A feature of the secondary animals is that during the development of the embryo at the site of primary mouth formation, anus forms and the mouth subsequently appears in a completely different place. In other words, we can say that the embryo has a mouth opening at one end, and the adult individual is in the opposite place. Secondary animals belong to the subsection of the kingdom, which includes echinoderms, chordates and half-chords. They are referred to as 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 to understand this? You just need to imagine a plane that, as it were, divides the body of the animal in half. In this case, both of these parts will completely coincide. In some sources, you can find the concept of "bilaterally symmetrical" animals.
This feature completely distinguishes this species from other representatives of the fauna and humans, in which the body is conditionally symmetrical. This means that not all organs are located on the same plane. But bilaterally symmetrical other planes do not. This feature has positive aspects. Such animals move very easily in straight lines and turn. These include primary and secondary animals.
The difference between primary and secondary
Representatives of these species, although similar, but there are still differences. As was said, bilaterally symmetrical include primary and secondary animals. These names come from how their mouth opening develops during embryonic development. In the primary, the blastospore (opening in the primary intestine) passes into the formation of the oral cavity. And the secondary anus forms in this place. In this case, the mouth opening is formed in a new way at the front end of the embryo. There are also examples when there is a complete closure of the blastopore, and the mouth and anus appear again.
And another important difference is the development of the primary brain. Primitives develop the brain of an adult animal. In the secondary, its reduction occurs, and a new one is formed again in another place. Secondary animals are also called secondary brain animals.
Classification of secondary animals
Above, we examined who secondary animals are, examples and features of their development. Now it's time to find out who belongs to this kingdom. These include the following types:
- chordates;
- maxillofacial;
- echinoderms.
Now let's look in detail which animals are secondary animals. Chordates include lancelet, lampreys, fish, amphibians, reptiles, birds and mammals. Marine animals belong to the maxillofacial, the most famous representative of which is the arrow of the sea. A very unusual type of echinoderms includes starfish, sea urchins, sea cucumbers, sea lilies. All of these fauna representatives 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.
Features of the development of the chord embryo
Chordates are secondary-rotated animals that differ in appearance, image and living conditions. Representatives of this type can be found everywhere. They live on land, and in water, and in soil, and in the air. Distributed all over the globe. The number is about forty thousand.
They are all united by the presence of an axial skeleton, which at the stage of embryo development is presented as a dorsal string (chord). In adults, it remains unchanged only in the lower representatives of the type. In all the others, it goes into the formation of the spine, which from a continuous cord turns into a segmented one.
The method of crushing a fertilized egg is also a distinguishing feature between these two kingdoms: spiral in the primary and the radial in the secondary.
The nervous system is a hollow tube, the front of which subsequently becomes the brain. From the inner cavity, its ventricles form.
In the anterior section of the digestive tube there are two rows of holes through which communication with the external environment takes place. These are the so-called visceral clefts. The lower representatives of the chordates in this place have gills. In all the rest, these are only germ embryos, which subsequently do not function.
Some sources are attributed to the secondary of the so-called semi-chord. These are vermiform bottom animals. They are characterized by the presence of notochords (chord-like organ) and paired gill slits. In embryonic development, they resemble chordates, but the body structure is completely different. The body is represented by three departments: proboscis, collar and torso.
Maxillary animals
These animals are marine predators with the ability to move quickly. Outwardly, they look like an arrow, which is pointed from the front end, and has a plumage at the rear. These are the very bristles with which the animal captures food. The body consists of a 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 circulatory, respiratory and excretory systems. There are also no reproductive ducts. The nervous system is a periopharyngeal ring.
Features of echinoderms
A characteristic feature of representatives of this type is the presence of an ambulacral system. These are fluid-filled cavities, thanks to which the animal moves, breathes, perceives 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. The sensory organs and nervous system are poorly developed. But regenerative abilities are well developed. In case of danger, these animals can discard individual parts of the body, which are restored after two weeks. Due to this feature , starfish can even multiply in half. After a while, the second half is fully restored.
Summary
From the foregoing, you can find out which animals belong to the secondary, about the features of their development and representatives of this kingdom. Obviously, representatives of this species are very interesting. Their research is still ongoing.