The field dedicated to categorizing living beings is known as taxonomy. All known species have been grouped into five kingdoms, and among these, there is the Animal Kingdom, where organisms are further categorized into various Phyla, Classes, Orders, Families, and Genera based on distinct identifiable characteristics.
One of the most fundamental forms of classification of animals is the presence or absence of the notochord. Hence, two major groups exist, namely: Chordates and Non-chordates.
The notochord is a pliable rod composed of a cartilage-like substance. An organism is designated as a chordate if it possesses a notochord at any stage of its life. It's important to note that chordates extend beyond vertebrates, contrary to common misconceptions. Some invertebrates have a notochord at some point in their life cycle, qualifying them as chordates. Therefore, while all vertebrates fall under the category of chordates, not all chordates are vertebrates.
Non-chordates are animals without a notochord – the rod-like elastic structure that supports the body. This phylum consists of a small group of worm-like, marine species with an organ-system level of organization. Members of phylum Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata and Hemichordata fall under Non-chordates.
The general characteristic features of Non-Chordates are:
Chordates are animals characterized by the presence of notochord at some stage during their development. Members possess a hollow nerve cord and pharyngeal gill slits. The other general characteristic features of Chordates are as follows:
Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata.
The Vertebrata subphylum is further subdivided into five distinct classes of vertebrate animals. These five classes encompass all animal species with a developed vertebral column and an internal skeleton. To date, more than 66,000 vertebrate species have been identified within the Chordata phylum. Vertebrates are characterized by their bilateral symmetry, possession of a coelom, triploblastic organization, and intricate differentiation of body tissues and organs.
Other characteristic features of vertebrates are:
They are aquatic animals, having a streamlined body and a pair of fins which are used for propulsion and movement. Furthermore, fish are cold-blooded, but the discovery of a new species in 2015 has changed this perception. The opah or the moon-fish is a fully warm-blooded fish capable of regulating its body temperature. Endoskeleton may be cartilaginous or bony and respiration occurs through gills. They do not possess eyelids because the surface of the eye is to be kept moist all the time.
Examples of Class Pisces includes dogfish and Rohu.
They usually comprise those organisms which are cold-blooded and require an aquatic habitat to lay eggs. These organisms are mainly characterized by the two pairs of limbs, smooth and moist skin for respiration. They also possess protruding eyes which are protected by usually more than one pair of eyelids. (Frogs have 3).
Examples of Class Amphibia are frog, toad, and salamander.
Class Reptilia comprises those organisms which are ectothermic in nature (cold-blooded). They are characterized by osteoderms which form scales, bony plates or scutes on the skin. Reptiles also lack an external ear and some reptiles such as snakes are actually “deaf” and instead, pick up vibrations through the ground. Another amazing sense that only snakes possess is Thermoception. This means that snakes can see infrared radiation emitted by objects or prey.
Examples of Class Reptilia are Tortoise, Wall lizard, Snake, etc.
Most members have a streamlined body specially designed to offer low air resistance during flight. In such birds, the forelimbs are modified into wings, with the power coming from breast muscles. Feathers play important roles, from flight, thermal insulation to water-proofing. All members of this class are warm-blooded and are able to regulate their body temperature. Aves have beaks, which are used for various functions such as preening and feeding. Furthermore, birds are considered to be the living relatives of dinosaurs (evolved from a group of meat-eating dinosaurs called the theropods).
Examples of Class Aves are Parrot, Pigeon, Duck, etc.
These organisms are distinguished by the presence of mammary glands. They have two pairs of limb for walking, grasping, swimming, flying, etc. Digits are provided with claws, nails or hooves. Skin is covered by hair and they have an external ear called pinnae. They are warm-blooded animals.
Examples of Class Mammalia include monkeys, lions, bats, squirrels and humans.
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1. What are non-chordates and chordates? |
2. How are vertebrates classified? |
3. How does the classification of vertebrates help in understanding their evolutionary relationships? |
4. What are some examples of non-chordates? |
5. How does the presence or absence of a notochord impact the classification of animals? |
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