The science of classifying organisms is called taxonomy. Every species discovered so far are classified into five kingdoms – one among them is Kingdom Animalia or the animal kingdom. The members of kingdom Animalia are further classified into different Phyla, Class, Order, Family, and Genus based on certain identifiable characteristic features.
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.
Non-chordates and the Chordates
The notochord is a flexible rod made out of a material similar to cartilage. If an animal has a notochord during any stage of its life, it is classified as a chordate. Contrary to popular belief, chordates do not exclusively include vertebrates. There are invertebrates that possess a notochord during some point in their lives and hence, are classified as chordates.
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:
- They are cylindrical, triploblastic, coelomate, or pseudocoelomate animals.
- Respiration in these animals takes place through gills, trachea or body surface.
- Most of the times, sexes cannot be distinguished among the members.
- Modes of reproduction involve sexual and asexual
- Fertilization is external, though internal fertilization also occurs in some species.
- The body includes an open type of circulatory system and front side central nervous system.
Examples of this phylum include Balanoglossus and Saccoglossus.
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:
- They are bilaterally symmetrical, triploblastic, and coelomate with the organ-system level of organization.
- They are often referred to as Protochordates and are exclusively marine.
- They hold a post-anal tail
- The body includes a closed circulatory system.
- In some species of Phylum Chordata, the notochord is present only in the larval tail, and in some, it is present throughout their life from head to tail region.
Examples of this phylum include Ascidia, Salpa, Doliolum, Amphioxus or Lancelet.
Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata.
Subphylum – Urochordata – It is also referred to as Tunicata which are marine animals. The body of these animals is surrounded by a leathery covering (tunic). Larvae are free-swimming, the notochord is present only in the tail of larvae and after settling on the seashore, they get transformed into sessile adults. They are generally hermaphrodite.
Examples include – Ascidians, Doliolum, Oikopleura, etc.
Subphylum – Cephalochordata– It mainly consists of small fish-like marine animals in which the notochord are extended along the entire body and includes pharynx which is large with numerous gill- slits. These species have separate sexes.
Example include – Amphioxus or lancelet.
Subphylum – Vertebrata – In this subphylum, the vertebral column is mainly replaced by and includes a well-developed head. The brain is protected in a cranium and the endoskeleton may be cartilaginous or bony. They may be jawless or with jaws
The subphylum Vertebrata is divided into five classes of vertebrates (animals with backbones.
The five classes of vertebrates comprise of all the species of animals have developed vertebral column and internal skeleton. Overall there are about 66,000 species of vertebrates which represent the overwhelming majority of the phylum Chordata. These vertebrates are bilaterally symmetrical, coelomic, triploblastic, and segmented, with complex differentiation of body tissues and organs. Thus all vertebrates are chordates but all chordates are not vertebrates.
Other characteristic features of vertebrates are
- Presence of a true vertebral column and internal skeleton completely distributed muscle attachment points for body movement.
- A front side muscular heart with two, three or four chambers.
- Kidneys for excretion and osmoregulation
- A paired appendages which may be fins or limbs.
- Possess notochord during the embryonic stage.
Vertebrates are further grouped into five classes. They are:
Class Pisces (Fishes)
They are cold-blooded, aquatic animals, having a streamlined body and a pair of fins which are used for swimming. Tail fin helps in changing direction and exoskeleton is the form of scales. Endoskeleton may be cartilaginous or bony while respiration occurs through gills. They also have eyes without eyelids.
Examples of Class Pisces includes dogfish and Rohu.
They usually comprise of those organisms which are cold-blooded and are the habitat of the freshwaters or terrestrial. These organisms are mainly characterized by the two pairs of limbs, smooth and moist skin for respiration. They have protruding eyes protected by usually more than one pair of eyelids (Frogs have 3). Examples of Class Amphibia are frog, toad, and salamander.
They comprise of those organisms which are ectothermic in nature (cold-blooded). Most members have dry skin bearing scales or bony plates. They are characterized by the presence of a neck and the absence of the external ear. Digits are provided with claws.
Examples of Class Reptilia are Tortoise, Wall lizard, Snake, etc.
Class Aves (Birds)
They are warm-blooded animals having a streamlined body for lower air resistance during flight and their forelimbs are modified into wings. They are characterized by digits which are clawed and covered with scales, exoskeleton in the form of feathers and the presence of a neck and beak. Examples of Class Aves are Parrot, Pigeon, Duck, etc.
These organisms are distinguished by the presence of mammary glands, a body that is divided into Head-Neck-Trunk-Tail. Digits are provided with claws, nails, hooves. Most of the members in this species lack the external ear (also called pinna). They are warm-blooded animals.
Examples of Class Mammalia are Monkey, Man, Lion, Bat, Rat, Squirrel, etc.
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