The Chordate Characteristics and Classification

All the animals are classified on various criteria, viz. cellularity, body symmetry, number of germ layers, coelomic cavity, and method of coelom formation. Besides these criteria, animals are classified into two groups based on the presence or absence of Notochord. These groups are-

(A). Non-Chordates, also called Invertebrates 

(B). Chordates.

While the vertebrates comprise the greater part of phylum chordate, two small groups, Urochordata and Cephalochordata (Protochordate or nonvertebrate chordate), are united with them as they share some characteristics with the vertebrates. These characters are as follows:    

1. Chordates are characterized by the presence of Notochord in embryo or adult:

The Notochord (Greek noton, back, + Latin. chorda, cord) is an evolutionarily conserved structure first studied by Alexander Kowalevsky. This is located at the midline and defines the primitive axis of the developing embryo. The notochord precursor is called “chordamesoderm,” which begins as a rod of stacked cells converging at the embryonic midline. During segmentation, the chordamesoderm cells differentiate into two sub-populations, an outer sheath layer and an inner vacuolated cell layer. The outer cells secrete thick, peri-notochordal basement membranes composed of many extracellular matrix proteins, and the inner cells with large fluid-filled vacuoles occupy most of the cell volume. The inner cell layer exerts pressure on the outer basement membrane providing stiffness and mechanical strength. Thus, a hydrostatic skeleton is generated during development, setting an anteroposterior axis before morphogenesis.

           In most chordates, the Notochord becomes ossified in the vertebrae-forming regions and is involved in forming intervertebral discs; these organisms are called vertebrates.       

Fig. 1. Schematic representation of cross-sectional structure of Zebrafish Notochord

2. Chordates have dorsal and hollow nerve chord:

In the chordates, the central nervous system is hollow (except in tunicates/urochordates, which are hollow only in the embryonic stage), containing a single continuous cavity. It is situated dorsally and runs down. In vertebrate chordates, the dorsal nerve chord becomes the spinal cord, and the nerves are of two types emerging from the nerve tube by the dorsal or ventral root. In invertebrates, the nerve chord is solid, paired, and ventral. In each segment of the invertebrate body, it swells to form ganglia.

3. Paired pharyngeal pouches (sacs) present in chordates:

In chordates, these are openings between the pharynx or throat and the outside. The pharyngeal cavity, called the pharyngeal pouch, through the mesodermal layer, reaches the ectoderm and forms pharyngeal or gill clefts. The pharyngeal pouch develops in air-breathing reptiles, birds, and mammals. The number of pouches in due course of evolution reduced from six or more to four in tetrapod and modified extensively. In primitive chordates, the slits are used for filter feeding. In fishes and amphibians, these clefts develop as gill slits and bear gills (used for gas exchange). In most chordates living on the land, these slits are present in the embryonic stage only.

4. All chordates have post-anal tail:

It is an extension of Notochord, chordate locomotor apparatus, and segmental musculature past the anus and represents posterior elongation of the body. In many chordates, like frogs and humans, the post-anal tail is present only in the larval or embryonic stage and not found in adults.

Fig. 2. Schematic representation of the chordate characters.

5. The heart is ventral in a chordate, and blood flows in the forward direction:

In the chordates, the heart is placed on the ventral side (except in cephalochordate, where the heart is absent), and blood flows in well-marked blood vessels. Blood flow is forwards ventrally and backward dorsally. In vertebrates, the circulatory system is closed, always with a median dorsal artery, the aorta, and one or two portal systems. In invertebrates, the direction of blood flow is reversed to that of the vertebrate condition, i.e., forwards dorsally and backward ventrally; otherwise, in invertebrates, the circulatory system is open type.

There are some other chordate (vertebrate) characters which are as follows:

1. Endoskeleton is the primary support system:

The internal supporting system of the hardened material is called the endoskeleton. In chordates (mainly vertebrates), the endoskeleton is comprised of bone or cartilage, or both. In invertebrates, the exoskeleton is the hardened supporting system.  

2. Higher forms have paired appendages:

The higher chordates (vertebrates) have paired fins or limb appendages derived from several segments. Sometimes, unpaired appendages are also present (as dorsal, caudal, and anal fins in fishes). In invertebrates, it is derived from one segment only.

3. The skin produces an integumentary system:

The skin is separable from the rest of the body wall and commonly produces protective structures, such as scales, feathers, hair, etc., which form the integumentary system.

4. The pharynx is connected to the respiratory system:

The pharynx either opens outside through gill slits or gives rise to an outgrowth to the lungs. The gills are born into the walls of the gill slits.

5. Genital and excretory systems are closely related:

The excretory duct generally serves as the genital duct. They have a common opening in the cloaca or separately near the anus. 

Chordates share many characteristics with higher non-chordates phyla. These are as follows:

1. Bilateral symmetry:

The body can be divided into two mirror halves through one and only one plane—organisms of phyla Platyhelminthes, Annelida, Mollusca, Arthropoda, and Chordata. Echinoderms are radially symmetrical, but their larvae are bilateral.

2. Coelom:

A coelom is a body cavity that forms a tube within a tube arrangement in coelomates (organisms with coelom). The coelom accommodates organs associated with the digestive system. All organisms of phylum Annelida, Mollusca, Arthropoda, Echinodermata, and Chordata are coelomate.   

3. Metameric segmentation:

The serial repetition of similar body parts along the anteroposterior axis of the body is called metameric segmentation or metamerism. Each segment of the body is called a metamer or somite. Annelids are metamerically segmented from the outside (body wall and setae), and many internal systems (muscle, blood vessels, nerves, gonads) also show metamerism. Proper segmentation is found only in three groups Annelida, Arthropoda, and Chordata.

4. Cephalization:

The concentration of nervous tissues and sense organs in the head region is called cephalization. It is the highest degree of specialization found only in bilaterally symmetrical organisms. 

Classification of Chordates:

All animals have evolved to survive and reproduce, employing different structures and strategies, but they have many things in common. These common themes provide essential insights into the evolutionary history of these organisms.

The Eukarya consists of Protozoa (unicellular) and Metazoa (multicellular). The Metazoa origin dates back to 770-850 million years. The simplest metazoan phylum is Porifera having a cellular level of organization. Cnidaria (jellyfish) and Ctenophora (comb jellies) have tissue level of organization, and two germ layers (Ectoderm and endoderm) are present; therefore, these are called diploblastic. Others have an additional tissue layer called mesoderm that lies in between the ectoderm and endoderm. These organisms are called triploblastic. They have bilateral body symmetry. The triploblastic gives rise to two separate lineages, Protostomes, and Deuterostomes. The protostome can be Ecdysozoans (Arthropoda and Nematode) or Lophotrochozoans (Mollusca and Annelida). The deuterostomes give rise to the Echinoderms, Hemichordates (Acorn worms), and Chordates (Urochordates, Cephalochordates, and Vertebrates).

The following flow chart shows the classification of phylum Chordata.

Fig. 3. Flow chart representing classification of Phylum Chordata


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