What distinguishes the second-term and primary animals

When describing the development of living organisms, one often encounters the terms "primitive" and "second-term animals". These are all existing multicellular, except coelenterates, which differ at the embryonic level.

Development of chordates

Chordates are represented by three groups of organisms:

• Shells, or larva-chordal (Tunicata, Urochordata), which have a baggy body enveloped in a shell;
• Cranial, or head-chord (Acrania), living in the depths of the sea, fish-like inactive beings;
• Vertebrates - representatives of fish, amphibians, snakes, birds, animals and humans.

The history of the origin of chordates is shrouded in mystery and causes a lot of assumptions. And in order to specify them, it is necessary to understand who are the second-term and primary animals. It is well known that more than 500 million years ago, there were already holo-chords. The preceding stages remain mysteries, and therefore different hypotheses arise about the origin of living beings.

The essence of primary and secondary animals

Multicellular represent the kingdom of animals. Primitive and second-term - their direct ancestors. Their verbal designations Protostomia and Deuterostomia are formed from the features of the formation of the mouth in embryos. In the former, it is formed from the opening of the primary intestine (blastopore), which is formed in the embryo. In the second - in the blastopore an opening hole (anus) develops, and the mouth opening is cut in a new place.

Classification of bilaterally symmetric organisms

Secondary and primary animals have their classifications. The first-born include:

1. Molluscs.
2. Ringworms.
3. Sipunculides - worm-like marine animals, which have the presence of an embryonic circulatory system, excretory system and a loop-shaped intestine with an anus on the side of the back.
4. Ehiurids are organisms living in sea waters. They have a cylindrical body with a proboscis. The oral opening is located on the peritoneal side. From other representatives they are distinguished by the presence of an underdeveloped circulatory system and a tubular intestine, which ends with an anus.
5. Pyatoustkov or reed, which are vermiform parasitic invertebrates with a linguiform body, similar to arthropods.
6. Onihoforov or primary-tracheal - are represented by terrestrial predators, on the long body of which there are up to 43 pairs of legs.
7. Pacific - small organisms with four pairs of limbs.
8. Arthropods.

Secondary animals, or deuterostomes, are represented by the following animal types:

• Bristle-mandibular;
• Semi-chordate (nadal and vasculitis);
• Echinoderms;
• Pogonophores;
• Chordate;
• Graptolites (fossils).

Stages of development deystorostom incomprehensible. It is assumed that their evolution began with multicellular invertebrate animals that do not overlap with the primary ones. Others believe that the ancestors of the secondary-worms are the lower worms, which are classed as primitive types of primary moles.

What is the difference between primary and secondary?

In addition to the formation of the entrance opening represented by the mouth, the second-term and primary-animals are distinguished by a number of other characteristics:

1. For a number of egg divisions: in the first they are radial, directed along the radial, and in the second - spiral (non-uniform).
2. Methods of laying the coelom (cavity): in primary growth, the development of the walls of the secondary cavities begins with cell division, while in the second-term ones they are formed from the protuberances of the intestinal pockets of the embryo.
3. The subsequent genesis of the brain: in the primary - it develops into the adult state, and in the second - it decreases and is laid in a new area.

Therefore, the second-term ones are also called secondary-cerebral beings.

Scientists believe that the second-term and first-term animals have one common ancestor, inhabiting the Edicar Sea more than 500 million years ago. The habitat of the creature was the seabed, by which it moved, working with cilia, located a strip on the abdomen, and fed by means of tentacles, which selected food. Perhaps later the posterior part of the body separated and the ancestors of the second-term used it to temporarily dig in the soil layer.

Secondary and primary animals are similar in development and structure. But they also have a number of differences, so that their followers differ.