When describing the development of living organisms, one can often find the terms "primary and" secondary animals. These are all existing multicellular, except for intestinal, which differ at the embryonic level.
Development of chordates
Chordates are represented by three groups of organisms:
- tunicates, or larval chordates (Tunicata, Urochordata), which have a saccular body wrapped in a shell;
- disembodied, or cephalopod (Acrania) living in the depths of the sea fish-like sedentary creatures;
- vertebrates - representatives of fish, amphibians, snakes, birds, animals and humans.
The history of the origin of chordates is shrouded in mystery and gives rise to many assumptions. And in order to concretize them, you need to figure out who these secondary and primary animals are. It is reliably known that more than 500 million years ago cephalic chordates already existed. The previous stages remain puzzles, so there are various hypotheses on the origin of living things.
The essence of primary and secondary animals
Multicellular animals represent the kingdom of animals. The primary and secondary are their direct ancestors. Their verbal designations Protostomia and Deuterostomia are derived 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, the excretory opening (anus) develops in the blastopore, and the oral opening is cut in a new place.
Classification of bilaterally symmetrical organisms
Secondary and primary animals have their own classifications. The primary ones include:
- Shellfish.
- Annelids.
- Sipunculides - worm-shaped marine animals in which the presence of an embryonic circulatory system, excretory system and loop-shaped intestine with anus from the back is revealed.
- Ehiurids - 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 tubular intestine, which ends with the anus.
- Pentate or reed, which are worm-like parasitic invertebrates with a linguistic body, similar to arthropods.
- Onychophora or primary tracheal - represented by terrestrial predators, on a long body of which there are up to 43 pairs of legs.
- Tardigrades - small organisms with four pairs of limbs.
- Arthropods.
Secondary animals, or deuterostomy, are represented by the following types of animals:
- maxillofacial;
- semi-chordate (peri-intestinal and pterygopalatine);
- echinoderms;
- pogonofory;
- chord;
- graptolites (fossils).
The stages of deuterostom development are incomprehensible. It is believed that their evolution began with multicellular invertebrate animals, which did not intersect with primary ones. Others believe that the ancestors of the secondary-worms are lower worms, which are classified as primitive types of primary-worms.
What is the difference between primary and secondary
In addition to the formation of the inlet, represented by the mouth, secondary and primary animals differ in a number of other characteristics:
- According to a number of egg divisions: in the first they are radial, directed along the radial, and in the second they are spiral (uneven).
- By the methods of laying coelomata (cavities): in the primary, the development of the walls of the secondary cavities begins with cell division, and in the secondary, they are formed from the protrusions of the intestinal pockets of the embryo.
- The subsequent genesis of the brain: in the primary, it develops into an adult state, and in the second, it decreases and is laid in a new area.
Therefore, the secondary is also called secondary cerebral creatures.
Scientists believe that secondary and primary animals have one common ancestor who lived in the Edicar Sea more than 500 million years ago. The creature's habitat was the seabed, along which he moved, working with cilia located on a strip on his abdomen, and fed through tentacles with which he picked up food. Perhaps, the back of the body later became isolated and the second-forefather ancestors used it to temporarily dig into the soil layer.
Secondary and primary animals are similar in development and structure. But they also have a number of differences, due to which their followers differ among themselves.