The classification of the animal kingdom is basically done on the basis of some characteristics. So, today in this article we will be discussing the 14 different characteristics for the classification of the animal kingdom.
Basis for Classification of Animal Kingdom
- Levels of organization
The first and most basic characteristic for the classification of the animal kingdom is the level of organisation of the cells that are present in the body of an organism. It is of three types-
i. Cellular Level
Animals are multicellular yet there is a difference in the pattern of organisation of cells. For example, sponges have loosely arranged cell aggregates, i.e., they lack the cellular level of organisation. There is a division of labour (activities) among the cells to some extent.
ii. Tissue Level
As we move to higher organisms like coelenterates and ctenophores, the arrangement of cells becomes more complex. The cells that have the same function are arranged into tissues, hence it is called as the tissue level of organisation.
iii. Organ Level
Platyhelminthes and other higher phyla have a higher level of cell organisation, i.e., organ level is present. The tissues having a similar function are grouped together into organs. In animals of phylum Annelids, Arthropods, Molluscs, Echinoderms and Chordates, the organs have associated to form a functional system. The systems are concerned with a specific physiological function. This pattern is called an organ system in the level of organisation. For example, the digestive system in Platyhelminthes has only a single outward opening of the body which is common to both mouth and anus and is hence called as incomplete. A complete digestive system has two separate openings, each for mouth and anus.
- Body Plan
The second basic characteristic for the classification of the animal kingdom is the body plan of an organism. Animals have various organs in their body that function together. Though these organs are diverse in shape and size, all the animals have a body type that falls into the three basic plans:
i. Cell Aggregate
Cell aggregates are clusters of cells that have a rudimentary type of labour division among them. The cells have less or no coordination among them. There are no tissues or organs. This type of body plan is present in sponges.
ii. Blind Sac
Blind sac type of body plan has only one opening in the alimentary canal common for both mouth and anus. There is tissue or organ system organisation in the animals. E.g. Coelenterates and Platyhelminthes.
iii. Tube within Tube
Tube-within tube type of body has the digestive system as a continuous tube that opens at both the ends. It is present in Nemathelminthes, Annelida, Arthropoda, Mollusca, Echinodermata and Chordata.
The third and one of the most important characteristics of the classification of the animal kingdom is the symmetry of an organism. The animals can be categorised on the basis of their body symmetry into the following three types:
i. Radial Symmetry
When the organism’s body is divided in any plane and it passes across the central axis of the body dividing the organism into two identical halves, it is called radial symmetry. The animals with radial symmetry are present in the group Radiata. For example, cnidarians (hydra and jellyfish). Biradial symmetry is also present in the sea anemone, ctenophores.
ii. Bilateral symmetry
Those animals having bilateral symmetry are put in group Bilateria. The organism’s body has a central axis which can cross by one plane and divide it into identical right and left halves in only one plane. For example, Platyhelminthes, annelids, arthropods etc. (Platyhelminthes to chordates).
Asymmetric organisms have a body that cannot be divided by any plane and thus there are no two equivalent halves. Sponges are mostly asymmetrical.
- Body Cavity or Coelom
An important factor for classification of the animal kingdom is the presence or absence of cavity between the body wall and gut wall.
The animals which do not have the coelom are called acoelomates, for example, poriferans, coelenterates, ctenophores, flatworms. Flatworms have the spaces between various organs that are filled with special tissue termed as parenchyma.
A body cavity is called as pseudocoelom if the body cavity has an incomplete lining of mesoderm. Instead, the mesoderm is like scattered pouches present in between the ectoderm and endoderm layers. e.g. in roundworm.
The true coelom in a body cavity arises as a cavity in embryonic mesoderm of the embryo that provides a cellular lining i.e. coelomic epithelium or peritoneum around the cavity. The coelom has coelomic fluid which is secreted by the peritoneum. A true coelom is found in annelids, echinoderms and chordates. A true coelom is of two types:
(a) Schizocoelom: Developing through the splitting of mesoderm. It is found in annelids, arthropods and molluscs. Body cavity in arthropods and non-cephalopod molluscs is thus called as heamocoel.
(b) Entericoelom: The mesoderm arises from the embryonic gut wall or enteron like hollow outgrowths or enterocoelomic pouches. It is found in echinoderms and chordates.
- Germ Layers
The fifth basic characteristic for the classification of the animal kingdom is the presence of germ layersin an organism. Germ layers are the first formed layers in the body which give rise to all the tissues/organs of the individual. Animals can be classified on the basis of the number of germ layers in the body.
The body cells are arranged in two layers – outer ectoderm and inner endoderm with an intervening undifferentiated mesoglea. E.g., Coelenterates.
The animals in which body cells are arranged in three germ layers, namely ectoderm, mesoderm and endoderm are called triploblastic animals. E.g., Platyhelminthes to chordates.
- Respiratory System
The sixth and one of the important characteristic of the classification of the animal kingdom is the respiratory system of an organism. The animals in sponges and coelenterates have each cell in the body in direct contact with the surrounding water and thus exchange the gases through their body surface. Higher animals have a thicker body wall and so they have organs that are dedicated to the respiratory work for the body.
- Blood Vascular System
The seventh and another important characteristic for the classification of the animal kingdom is the blood vascular system of an organism that it posses. Blood vascular system is basically classified into two types: open blood vascular system and closed Blood vascular system.
The eighth basic characteristic for the classification of the animal kingdom is the presence of segments in the body of an organism. Some animals have the body externally and internally divided into segments with a serial repetition of at least some organs. For example, earthworm’s body shows this pattern called the metameric segmentation and the phenomenon is known as metamerism. Arthropods, annelids and chordates also have metameric segmentation.
Pseudo metameric – e.g. Tapeworms
Metameric – In Annelids, arthropods and chordates.
The ninth and the most important characteristic for the classification of the animal kingdom is the presence of notochord in an organism. Some animals have a Notochord, developed during the embryonic growth, is a mesodermal rod-like structure formed on the dorsal side. Animals that possess notochord are called as chordates and those animals which lack notochord are called as non-chordates, e.g., Porifera to echinoderms.
- Excretory System
Nitrogenous wastes are excreted out from the body which if not excreted out regularly can imbalance the homoeostasis inside the body. The system who takes care of the excretion is called an excretory system. The excretory organs are dedicated for excretion and osmoregulation (to maintain the water level in the body). On the basis of nature of main nitrogenous waste, excretion is of three types:
The major nitrogenous waste excreted is NH3 in the animals which are called as ammonotelic animals. Aquatic animals excrete ammonia e.g. sponges, coelenterates, ctenophores, crustaceans, echinoderms and bony fishes.
The major nitrogenous waste excreted out is urea from the animals which are called as ureotelic animals. E.g. amphibians, mammals and cartilage fishes.
The major nitrogenous waste excreted is uric acid from animals which are called as uricotelic animals. E.g. reptiles, birds and insects.
- Nervous System
The eleventh basic characteristic for the classification of the animal kingdom is the nervous system of an organism. The body functions are in control and coordination with the organs in the body to maintain homeostasis inside the body. Some organisms lack this system like sponges. There are different types of nervous system in different groups of animals The type of nervous system found in different animal groups are-
The ultimate function is the reproduction in all the living organisms and is essential for the species to exist in the world. Reproduction is sexual and asexual. Several species use asexual methods to reproduce. e.g. Binary fission (parent divides in two daughters in favourable conditions e.g. Amoeba); Multiple fission (parent divides into many daughter cells in unfavourable conditions e.g. Amoeba); Budding (parent develops an external or internal outgrowth called as buds that grow in new organism e.g. Hydra) and fragmentation.
The other common method is sexual reproduction (which involves formation and fusion of gametes). Gametes are the sex cells formed in the sex organs called the gonads. Gonads are different, the male has testes and female has ovaries while sex cells are called as sperms and ova respectively. The two types of gonads can be present in different animals which are called as unisexual or dioecious e.g. human beings, frog, cockroach etc. The two sexes have an extremely different structure called as sexual dimorphism e.g., peacock and peahen, human beings etc.
However, some animals have both the types of gonads (testes and ovaries) in the same animal, called as bisexual or monoecious or hermaphrodite e.g., earthworm, leech, Taenia etc. Sperm and ovum fuse to form zygote called fertilisation. Fertilisation can be external (e.g. echinoderms, bony fishes and amphibians) or internal (e.g. land vertebrates and cartilage fishes) if fusion is outside or inside the female. The sperms and ova are derived from different animals which are called as cross-fertilization. In some of the bisexual animals, sperm and ovum from the same animal fuse and form zygote, which is called as self-fertilization e.g., in Taenia. Cross-fertilization occurs in two ways: protandrous and protogynous.
In protandrous condition (proto = first, androus = male), testes mature first followed by the ovaries e.g., leech, earthworm etc. In protogynous condition (proto = first, gynae = female), ovaries mature first than the testes. e.g., Scypha, Herdmania (Sea squirt).
- Body Temperature
The body temperature varies in animals, thus they are divided, into two categories:
(a) Homeothermal (Warm-blooded or endothermic animals): the body temperature is constant throughout in any environmental temperature e.g., birds and mammals.
(b) Poikilothermal (Cold-blooded or ectothermic animals): the body temperature changes as the
environmental temperature changes. E.g. amphibians and lizards.
The structure of the body which gives it the shape, support and protection is called a skeleton. The skeleton is of two types:
(a) Endoskeleton: It is formed of living structure and is present inside the body e.g., cartilages and bones. e.g., vertebrates.
(b) Exoskeleton: It is formed of dead structures always covering the outside body. e.g., chitinous plates called sclerites in arthropods; calcareous shell in molluscs; epidermal scales in reptiles; epidermal feathers in birds; epidermal hair, nails, claws, horns and hoofs in mammals.