Chordata

Chordates (Chordata). The animals of this phylum belong to the Deuterostomes branch of the animal kingdom , that is, during embryonic development the blastopore (the first opening of the embryo) gives rise to the anus. In this deuterostome branch, the Echinoderms , the Hemichordates , the Urochordates and the Lophophorate phyla ( Phoronídeos , Ectoprocts and Brachiopods ) are also present. Approximately 50,000 living species belong to this group (which also represent 1/10 of the extinct species). In addition, they present great morphological, behavioral and ecological diversity. Cordados comes from the Latin Chorda which means “cord”.

Summary

[ disguise ]

• 1 General characteristics
• 2 Phylogeny
  • 1 Subphylum Urochordates (Tunicates)
  • 2 Subphylum Cranians (Vertebrates)
• 3 Bibliographic References
• 4 Sources

General characteristics

Among the characteristics that chordates present are:

• Presence of Coelom (internal cavity filled with fluid).
• Enterocoelom (layers of mesoderm form pockets from the TD to form the body cavity).
• Deuterostomes (second mouth), the mouth arises not from the blastopore but secondarily from the opposite end of the embryonic digestive tract.
• Radial cleavage.
• Calcified skeleton that when present is generally derived from mesodermal tissue.

The unique characteristics that allow us to recognize this group of organisms from other deuterostome coelomates are the following:

1. Notochord:longitudinal axial rod formed by a wall of connective tissue fibers around large cells distended by fluid-filled vacuoles.

• Elongated, flexible rod (that does not collapse) that arises from the dorsal wall of the embryonic TD. It is located dorsal to the coelom but below and parallel to the central nervous system .
• Cell center and fluid (usually in cellular vacuoles or between cells ) encapsulated in a hard layer of fibrous tissue.
• In the evolution of the group it is replaced by the vertebral column , although it is retained at the embryonic level.

2. Dorsal and hollow nerve cord:unlike invertebrates where it is ventral and not hollow.

• Derived from the ectoderm by a process of invagination. The future tubular nerve cells of the chordate embryo assemble dorsally into a thickened neural plate within the surface of the ectoderm.

3. Pharyngeal slits or pouches:although they are characteristic of chordates, they are not unique to chordates, since we find similar structures in members of the phylum Hemichordata, and even in some echinoderm fossils.

• They appear early in embryonic development and may persist into the adult state, or disappear before birth or hatching. In early evolution, feeding function, later they were associated with respiratory tissue (gills), they also began to participate in respiration. In basal groups the Pharynx expands into a pharyngeal basket and the slits and their walls multiply in number, increasing the surface area exposed to the flow of water.

4. Postanal muscular tail.

• Posterior elongation of the body that extends beyond the anus.

Primarily an extension of the locomotor system, the segmented musculature and the notochord.

5. Endostyle

• Present in larval Lampreys and adult Prochordates.

Ciliated structure, glandular slit in the floor of the pharynx that secretes mucus and is used to trap food in filter feeding. Homologous with the thyroid gland of vertebrates (the transformation is seen directly in the metamorphosis of the lamprey). Both concentrate iodine (from surrounding water or blood ) . In Cranians (larger animals) this system is inefficient and water currents are driven by muscle.

These characteristics may be present only briefly during embryonic development or may persist into the adult stage. The clear thing is that chordates always exhibit them at some point in their lives.

In addition, chordates present bilateral symmetry and segmentation. That is, blocks of muscle (myomeres) are arranged sequentially along the body and tail as part of the external wall of the body.

Phylogeny

Phylogeny.jpg

The fossil record does not preserve direct evidence of intermediate forms between chordates and other animals. However, we can use the diversity of living animals and the characteristics (morphological and genomic) they share to infer the origin of chordates. The latest evidence at the molecular level indicates that the chordates would be made up of two groups: the Urochordates (Tunicates) + the Cranians ( Vertebrates ). While the Cephalochordates, previously located within the chordates, would be more related to the Echinoderms.

Subphylum Urochordates (Tunicates)

They are common marine animals that feed by filtering water. Most are sessile and the body is covered by a complex exoskeletal tunic. They have a highly developed perforated pharynx, but in the adult, the notochord and nerve cord generally disappear and only the larval stages, which resemble microscopic tadpoles, possess the distinctive characteristics of chordates. About 1,250 species of urochordates have been described so far.

Within the group three classes can be recognized:

• Ascidiacea .
• Thaliacea .
• Larvacea .

Subphylum Cranians (Vertebrates)

Vertebrates constitute the largest and best-known subgroup of chordates. Certain characteristics that they present in the cephalic region, such as the concentration of sensory organs and the brain , clearly distinguish vertebrates, even the most primitive ones, from non-vertebrate chordates. It has been suggested that the transition from a passive mode of feeding by ciliary filtering to an active one was accompanied by changes in muscle activity and cephalization that differentiate the ancestors of vertebrates from current vertebrates.

Most vertebrates have a column or spine as their structural axis; However, the most primitive forms lack vertebrae. The vertebral column is a flexible support, usually bony, that develops around the NOTOCORD, completely supplanting it in most species. The dorsal projections of the vertebrae surround the nerve cord along the spine. The brain is similarly enclosed and protected by a skull . Between the vertebrae there are cartilaginous discs that give flexibility to the spine. In association with the vertebrae, there are segmented muscles by means of which sectors of the spine can move separately. This segmented pattern persists in the embryonic forms of higher vertebrates, but is completely lost in the course of development.

One of the great advantages of a bone endoskeleton is that it is made up of living tissue that can grow with the animal. In the developing vertebrate embryo, the skeleton is mainly cartilaginous; In most vertebrates, bones gradually replace cartilage in the course of maturation. The growing portions of the bones characteristically remain cartilaginous until the animal reaches its final size. There are seven living groups of vertebrates: agnathous, bony and cartilaginous fishes, amphibians, reptiles, birds and mammals. Its evolution is clearly documented in the fossil record.

Within the group you can find five classes: