6. Animal Classification

You have studied the classification of living organisms in earlier classes. The living organisms present around us are mainly plants and animals. We have studied the criteria of their classification. With the help of that, complete the following chart.

We have studied the plants classification in last year. It helped us to understand the diversity of plants around us. You must be looking varieties of animals around you. Some animals are too small whereas some are too big. Some animals are terrestrial where as some are aquatic. Some animals crawl on land, some swim in water whereas some fly in air. Some animals have scales on skin whereas some have feathers or hairs. In this way, there is huge diversity among the animals too. According to recent studies, estimated number of animal species on earth is approximately 7 million. It is impossible to study each and every species. However, if groups and sub-groups of animals are formed depending upon the similarities & differences, it will make it very easy to study such vast variety of animals.

History of animal classification

Time to time, different scientists have tried to classify the animals. Greek philosopher Aristotle was the first to perform the animal classification. Aristotle classified the animals according to the criteria like body size, habits and habitats. Further, as per the new developments in sciences, references were changed and thereby the criteria of animal classification too. Classification proposed by Aristotle is known as ‘Artificial method’. Besides Aristotle, artificial method of classification was followed by Theophrastus, Pliny, John Ray, Linnaeus, etc. Later on, ‘Natural system of classification’ was followed. Natural system of classification was based on various criteria like body organization, types of cells, chromosomes, bio-chemical properties, etc. By the time, system of classification based on evolution was also brought into practice. It was used by Dobzhansky and Meyer. Recently, Carl Woese has also proposed the animal classification

Traditional method of animal classification

Traditionally, depending upon presence or absence of the notochord, the animal kingdom has been divided into two groups- Non-chordates and Chordates.

Non-Chordates : Characters of non-chordate animals are as follows

  1. Body is not supported by rod-like notochord.
  2. Pharyngeal gill-slits are absent.
  3. Nerve cord; if present, it is on ventral side. It is solid & paired.
  4. Heart, if present, it is on dorsal side.

Chordates : Characters of chordates are as follows

  1. Body is supported by notochord.
  2. Pharyngeal gill-slits or lungs are present for respiration.
  3. Nerve cord is present on dorsal side of body. It is hollow.
  4. Heart is present on ventral side of body.

Notochord is a long rod like supporting structure present on dorsal side of animal body. It keeps the nerve tissue isolated from remaining body.

Do you know?

All chordate animals are grouped together in a single phylum and the name of phylum is same i.e. Phylum- Chordata. This phylum has been divided into three subphyla as- Urochordata, Cephalochordata & Vertebrata. Sub-phylum Vertebrata has been further divided into six classes as- Class: Cyclostomata, Class: Pisces, Class: Amphibia, Class: Reptilia, Class: Aves and Class: Mammalia.

This system of animal classification was in practice till now. However, now a days, new system of classification is followed. We will study this new system of animal classification in brief.

At present, according to the five kingdom classification system of Robert Whittaker, all multicellular animals are included in Kingdom: Animalia. This system of classification is based upon some criteria like Body organization, Body symmetry, Body cavity, Germinal layers, Segmentation, etc.

Criteria for new system of classification

Grades of organization

Body of animals is made up of cells. In case of multicellular animals, many cells are performing different functions in their body.

In case of multicellular animals, if tissues are not formed, their body organization is called as ‘Cellular grade organization’. Ex. Phylum-Porifera. In case of some animals, cells come together to form tissues with the help of which all the body functions are performed. Such animals show ‘Cell – tissue grade’ organization. Ex. Animals from phylum- Cnidaria.

Flat worms show ‘Tissue[1]Organ grade’ organization. In this type of organization, tissues are organized to form some organs. However, complete organ systems are not formed.

Besides the four types of body organizations mentioned above, Remaining all animals show ‘Organ – system grade organization’ in which different organs are joined together to form organ system that performs specific functions. Ex. Crab, Frog, Human, etc.

Body Symmetry

Take the pictures of human body and spongilla and try to take an imaginary section through specific plane of their bodies so as to get two equal halves. What did you observe?

In imaginary sense, if body of any animal is cut through imaginary axis of body, it may or may not produce two equal halves. Depending upon this property, there are different types of animal bodies.

Asymmetrical Body : In case of such body, there is no any such imaginary axis of the body through which we can get two equal halves. Ex. some sponges.

Radial symmetry : In this type of body, if imaginary cut passes through central axis but any plane of body, it gives two equal halves. Ex. Star fish. In case of this animal, there are five different planes passing through central axis of body through which we can get two equal halves.

Bilateral symmetry: In this type of body, there is only one such imaginary axis of body through which we can get two equal halves. Ex. Insects, fishes, frog, birds, human, etc.

Germ Layers: Diploblastic and triploblastic

In case of multicellular animals, germ layers are formed during initial period of their embryonic development and from those germ layers only, different tissues are formed in the body. In case of some animals, only two germ layers [Endoderm & ectoderm] are formed. Ex.: All Cnidarians. In most of all the remaining animals, three germ layers are formed i.e. mesoderm besides endoderm & ectoderm.

Body cavity (Coelom)

Cavity between the body wall and internal organs is called as body cavity/coelom. In case of multicellular animals, during initial period of their embryonic development, body cavity is formed from either mesoderm. Such type of body cavity is present in animals of phylum Annelida and all phyla coming after Annelida. Such animals are called as eucoelomate (animals with true body cavity). Body cavity is absent in case of animals from phyla Porifera, Cnidaria and Platyhelminthes. Such animals are called as acoelomate. In case of animals from phylum Aschelminthes, they have body cavity but it is not formed by the above mentioned two ways. Hence those animals are called as pseudocoelomates.

 Body Segmentation

If the body of animals is divided into small, similar units, then such body is called as segmented body and each small unit is called as segment. Ex. Animals like earthworm from phylum Annelida.

Phylum- Porifera

  1. These animals are with simplest body plan and are called as ‘Sponges’. They bear numerous pores on their body. Those pores are called as ‘Ostia’ and ‘Oscula’.
  2. These are aquatic animals. Most of them are marine and few are fresh water dwellers.
  3. Most of the animals have asymmetrical body.
  4. These animals have special types of cells- collar cells.
  5. These animals are always attached to substratum, hence do not show locomotion. Hence, they are referred as sedentary animals.
  6. Their spongy body is supported by spicules or spongin fibres. Spicules are made up of calcium carbonate or silica.
  7. These animals feed upon small organisms taken in their body along with water. Water is taken in through ostia and given out through oscula.
  8. These animals reproduce by budding, an asexual method and / or by sexual method. Besides, they have good ability of regeneration. Examples: Sycon, Euspongia (Bath sponge), Hyalonema, Euplectella, etc.

Phylum – Coelenterata/Cnidaria

  1. Body of these animals is cylindrical or umbrella-like. If it is cylindrical, it is called as ‘Polyp’ and if it is umbrella like, it called as ‘Medusa’.
  2. Most of these animals are marine. Only few are fresh-water dwellers.
  3. Body of these animals is radially symmetrical & diploblastic.
  4. Cnidoblast bearing tentacles are present around the mouth. Tentacles are useful for capturing the prey whereas cnidoblasts inject the toxin in the body of prey. Those are useful for protection too. Examples: Hydra, Adamsia (Sea anemone), Physalia (Portuguese- man-of war), Aurelia (Jelly fish), Corals, etc.

Phylum – Platyhelminthes

  1. Body of these animals is slender & flat like a leaf or strip. Hence, they are called as ‘flatworms’.
  2. Most of these animals are endoparasites. Few are free-living & aquatic.
  3. Body is acoelomate & bilaterally symmetrical.
  4. These are triploblastic i.e. their body is made up of three germ layers- endoderm, ectoderm & mesoderm.
  5. These animals are hermaphrodite i.e. male and female reproductive systems are present in the same animal body. Examples: Planaria, Liver fluke,Tape worm, etc.

Phylum- Aschelminthes

  1. Body of these animals is long thread-like or cylindrical. Hence, they are called as round worms.
  2. These animals are either free living or endoparasites. Free living animals are either aquatic or terrestrial.
  3. Body of these animals is triploblastic and pseudocoelomate.
  4. Body of these animals is non-segmented and covered with tough cuticle.
  5. These animals are unisexual. Examples: Ascaris (Intestinal worm), Filarial worm, Loa loa (Eye worm), etc.

Phylum – Annelida

  1. Body of these animals is long, cylindrical and metamerically segmented.
  2. Most of the animals are free-living, but few are ectoparasites. Free-living animals may be marine or fresh water dwellers or terrestrial.
  3. These animals are triploblastic, bilaterally symmetrical and eucoelomate.
  4. They have setae or parapodia or suckers for locomotion.
  5. Their body is covered with special cuticle.
  6. These animals are either hermaphrodite or unisexual. Examples: Earthworm, Leech, Nereis, etc

Phylum- Arthropoda

  1. These animals have jointed appendages. Hence they are called as arthropods.
  2. Planet Earth has highest number of animals from this phylum. Hence, this is largest phylum with highly successful animals in animal kingdom.
  3. These animals are found in all types of habitats ranging from deepest oceans to highest mountains.
  4. Body of these animals is triploblastic, eucoelomate, bilaterally symmetrical and segmented.
  5. Chitinous exoskeleton is present around their body.
  6. These animals are unisexual. Examples: Crab, spider, scorpion, millipede, centipede, cockroach, butterfly, honey bee, etc.

Phylum- Mollusca

  1. Body of these animals is soft and slimy. Hence they are referred as mollusc.
  2. This is second largest phylum in animal kingdom.
  3. These animals are aquatic or terrestrial. Most of the aquatic molluscs are marine, but few are fresh water dwellers too.
  4. Body of these animals is triploblastic, eucoelomate, non-segmented and soft. Except animals like snail, their body shows bilateral symmetry. Their body is divided into three divisions like head, foot and visceral mass.
  5. Visceral mass is covered with mantle. This mantle secretes a hard, calcareous shell. This shell may be external or internal or even absent in some cases.
  6. These animals are unisexual. Examples: Bivalve, Snail, Octopus, etc.

Phylum- Echinodermata

  1. Calcareous spines are present on the body of these animals; hence they are called as echinoderms.
  2. These animals are found only in ocean.
  3. Their body is triploblastic, eucoelomate. And it is radially symmetrical in adult stage. However, they show bilateral symmetry in larval stage.
  4. They perform locomotion with the help of tube-feet. Tube feet are also useful for capturing the prey. Some animals are sedentary.
  5. They have skeleton made up of calcareous spines and / or ossicles (plates).
  6. These animals have good ability of regeneration.
  7. These animals are mostly unisexual. Examples: Star fish, sea-urchin, brittle star, sea-cucumber, etc.

Phylum- Hemichordata

  1. Body of these animals is divided into three parts as proboscis, collar & trunk.
  2. Notochord is present in proboscis region only. Hence, they are called as hemichordates.
  3. These animals are also called as ‘acorn worms’.
  4. These are marine animals, live in burrows in sand.
  5. They have one to many pharyngeal gill slits.
  6. They are unisexual or some may be hermaphrodite. Ex.: Balanoglossus, Saccoglossus.

Phylum- Chordata

These animals have supporting notochord in their body. All chordates are included in the same phylum. The phylum Chordata is classified in to three subphyla. Following are important characters of phylum Chordata

  1. Notochord is present in the body during at least any developmental stage.
  2. Pharyngeal gill slits are present in the body during at least any developmental stage.
  3. Single, tubular spinal cord is present on dorsal side of body.
  4. Heart is present on ventral side of body.

 Sub phylum – Urochordata

  1. These are marine animals.
  2. Their body is covered by skin-like test or tunic.
  3. Larvae of these animals are freely swimming and notochord is present in only tail region of larvae. Hence, they are called as Urochordata.
  4. Larvae metamorphose into adults after settling down at bottom of the sea.
  5. Generally, these animals are hermaphrodite. Examples: Herdmania, Doliolum, Oikopleura, etc

Sub phylum -Cephalochordata

  1. These are small, fish-like, marine animals.
  2. Notochord is present throughout the body length.
  3. Pharynx is very large and contains gill-slits.
  4. These animals are unisexual. Ex.: Amphioxus.

Sub phylum -Vertebrata/Craniata

  1. In these animals, notochord is replaced by vertebral column.
  2. In these animals, head is well developed.
  3. Brain is protected by cranium.
  4. Endoskeleton is either cartilaginous or bony.
  5. Some chordates are jaw-less (Agnatha) whereas some are with jaws (Gnathostomata).

Subphylum- Vertebrata is divided into six classes as follows[1]

Class- Cyclostomata

  1. These animals have jaw-less mouth provided with sucker.
  2. Their skin is soft and without any scale.
  3. Paired appendages are absent.
  4. Endoskeleton is cartilaginous.
  5. Most of the animals are ectoparasites. Examples: Petromyzon, Myxine, etc.

Class- Pisces

  1. These are cold blooded (Poikilotherms) aquatic animals living in marine and fresh waters.
  2. Body is spindle shaped to minimize water-resistance.
  3. They have paired & un-paired fins for swimming. Tail fin is useful as a steering organ during swimming.
  4. Exoskeleton is in the form of scales & endoskeleton is either cartilaginous or bony.
  5. Respiration occurs with gills. Examples.: Rohu, Pomfret, Sea horse, Shark, Electric ray, Sting ray, etc.

Class- Amphibia

  1. These animals are strictly aquatic during larval life and perform only aquatic respiration whereas they can live in water as well as on land during adult life and can perform aquatic as well as aerial respiration.
  2. They have two pairs of appendages. Digits are without claws.
  3. Exoskeleton is absent. Skin is without any derivative and usually kept moist for respiration.
  4. External ear is absent but tympanum is present.
  5. Neck is absent. Eyes are prominent with eye lids. Ex.: Frog, Toad, Salamander, etc.

Class- Reptilia

  1. According to the course of animal evolution, these are first true terrestrial animals with creeping movement.
  2. These are cold blooded (poikilotherms) animals.
  3. They creep on the land as their body cannot be lifted up.
  4. Their skin is dry and scaly.
  5. Neck is present between head & trunk.
  6. External ear is absent.
  7. Digits are provided with claws. Examples: Tortoise, Lizard, Snake, etc.

Class- Aves

  1. These vertebrates are completely adapted for aerial life.
  2. These are warm blooded (Homeotherms) i.e. they can maintain their body temperature constant.
  3. Their body is spindle-shaped to minimize air resistance during flight.
  4. Forelimbs are modified into wings. Digits are covered with scales and bear claws.
  5. Exoskeleton is present in the form of feathers.
  6. Neck is present between head and trunk.
  7. Jaws are modified into beak. Examples: Peacock, Parrot, Pigeon, Duck, Penguin, etc.

Class- Mammalia

  1. Presence of mammary glands is typical character of mammalia.
  2. These animals are warm blooded.
  3. Body is divided into head, neck, trunk and tail.
  4. Digits are provided with nails, claws, or hooves.
  5. Exoskeleton is in the form of hairs or fur. Examples: Elephant, Human, Kangaroo, Dolphin, Bat, etc.