In this article, we will examine in detail the question of which features are common to chordates. Representatives of this type are distinguished on the basis of a number of characteristic features. We suggest that you familiarize yourself with the main ones.
So what are common signs for chordates? Let's start with bilateral symmetry. This symptom is one of the most important.
Bilateral symmetry
All chordal animals are characterized by bilateral (bilateral) symmetry. The same structure is characteristic of other types of multicellular animals, starting with lower worms. Bilateral symmetry reflects an important moment in the evolution of multicellular organisms. The transition to active movement in the environment is potentially associated with the intensification of nutrition and metabolic rate, the diversity of life forms and the expansion of the range of biotopes available for permanent residence.
Secondary body cavity (whole)
The second major stage in the evolution of animals was the formation of a secondary body cavity (coelom). This stage begins with annelids. The biological significance of the secondary body cavity is associated with further activation of movement and nutrition. In barren and primitive animals, the intestines are surrounded by loose parenchymal tissue or fluid, the movement of food in the digestive tract is carried out by contractions of the skin-muscular sac, which simultaneously cause translational movement of the whole organism. In other words, the contractions of the entire trunk and intestines are synchronized, which is not biologically always beneficial for the efficient absorption of food.
The emergence of the secondary cavity of the body, which separates the intestines and the musculocutaneous sac, and the appearance of the own intestinal muscles formed from the mesoderm, open up the possibility of intestinal motility independent of locomotion. Each of these important functions - movement in space and digestive activity - is carried out depending on environmental requirements, does not limit each other. Moreover, the whole can play a supporting role, acting as a "hydroskeleton".
No less important is another function of coelom - transport. Its outgrowths, penetrating deep into the tissues, provide them with a supply of nutrients and oxygen. Based on the outgrowths of the intestine, the circulatory system is formed. Excretory organs are also associated with the coelom . Thus, on the basis of the secondary cavity of the body, exchange is maintained at the tissue and organ level.
All chordates belong to secondary-celled animals, which phylogenetically connects them with types such as annelids, bryozoans, brachopods, arthropods, echinoderms, pogonophors, and others. Second-celled animals originate from ancient intestinal animals.
Secondary turnover
Considering the common signs of chordate animals (the subtype is inconspicuous), it is necessary to note the secondary turnover. All secondary-celled organisms decompose into two branches: primary and secondary. The names of the groups are associated with the peculiarities of embryonic development: in the former, the position of the mouth corresponds to the blastopore, which is divided into the mouth and anus, and in the second, the blastopore assumes the function of the anus, and the mouth erupts in another place. This group includes hemichordates, echinoderms, pogonophors and chordates. All other types of secondary-celled animals belong to primary-celled animals.
But the differences between these groups are more significant than the position of the mouth opening. First of all, they differ in the nature of the formation of coelom: in the majority of primary whales, the whole is formed schizotelously (by splitting the mesenchyme), and the mesoderm arises by migration of cells from adjacent tissues into the cavity (teloblastic type). In the secondaries, it is generally enterocele; it develops by paired intestinal protrusion: their walls give rise to the mesoderm leaf. In addition, the primary ones are characterized by an open circulatory system and the “staircase” type of structure of the central nervous system, while in the secondary ones the circulatory system is overwhelmingly closed, and the central nervous system has a different structure, in which large accumulations of nerve cells often form in separate places.
Chord specific features
In addition to the listed features common to all chordates, but found among other animals, representatives of the type we are interested in have some specific structural features. The main of them we will consider in more detail below.
Chord
All chordates have an internal axial skeleton, the main element of which is the chord. It is an elastic cord composed of vacuolated cells that form cartilaginous tissue of endodermal origin. The chord is surrounded by a connective tissue cover. Its main function is support; axial skeleton helps maintain body shape. The close relationship with the surrounding axial muscles and a certain degree of mobility, elasticity determine the participation of the chord in the lateral bends of the body, creating translational motion in a dense aqueous medium.
The chord, as the only structure of the axial skeleton, exists only among the lower representatives of the type; in most vertebrates, it is laid in the embryonic period of development, but is later replaced by the spine, which forms in its connective tissue membrane. In biology tests, the question is often asked: "What features are common to chordates and fish?" One of the correct answers is "the presence of a chord." In fish, the spine later assumes all its functions (including locomotor), and in terrestrial vertebrates, it is mainly supportive; his direct participation in locomotion is replaced by the support function for individual parts of the locomotion apparatus.
Tubular structure of the central nervous system
The central nervous system in the form of a tube with a cavity inside is a strictly specific sign of the chordate type. Due to the fact that the neural plate that is laid from the ectoderm during further embryogenesis coagulates into a tube, a cavity is created inside the spinal cord formed in this way - a neurocele (spinal canal) filled with cerebrospinal fluid.
We have not yet examined all the signs of chordates. Let's talk about one more.
Gill slits
A characteristic feature of the representatives of the type we are interested in is that the anterior section of the intestinal tube of these animals is penetrated by gill slots - the openings connecting the pharyngeal cavity (this section is called this) with the external environment. The appearance of gill slots is associated with the filtering nature of nutrition: water is released through them after separation of food particles that enter the intestine.
Finally
So, we talked about what signs are common to chordates. On the basis of them, as well as some other characteristics, representatives of this type are distinguished from others. It is necessary not only to memorize, but also to understand the general signs of chordate animals. The table below contains information on what subtypes and classes all of its representatives are divided into.
We hope the material presented in this article helped you understand the features of this type.