What is an Kingdom Animalia? Attributes, Pattern, Hierarchy & Classification

The Kingdom Animalia also known as Metazoa, comprises a broad range of animal species, from tiny parasitic nematodes and microscopic invertebrates to the largest of mammals — the blue whale. According to the University of Michigan Museum of Zoology, the exact number of members of kingdom Animalia is unknown and difficult to guess; there are over one million estimated insects alone.

Attributes of Animalia Kingdom

The animalia kingdom consists of various multicellular eukaryotic animals. It is one of the kingdoms among the five kingdom scheme of classification (by Whittaker).

What is the Basis of Classification?

There are a few important characteristics that are regular to different organisms. Therefore, it is important for us to know about the various components. These components include body symmetry, nature of coelom, the arrangement of cells, notochord, patterns of circulatory, digestive and reproductive frameworks, segmentation and an arrangement of cells in germ layers. Now we will look at these components in details.

Levels of Organization

Every organism of the Animalia kingdom is multicellular. However, they don’t display the same example of cell organization. The examples of cellular organization found in animals are:

• Cellular Level of Organization – In such animals, the cells are organized as free cell lumps. We can see this sort of cellular organization in sponges. Illustration: Sponges.
• Tissue Level of Organization – Animal cells shows the division of exercises amid themselves. Cells carrying out the same capacity are organized as tissues. Illustration: Coelenterates.
• Organ Level of Organization – We group the animal tissues carrying out similar capacity into shape organs. Every organ is specific for particular capacity. Illustration: Platyhelminthes.
• Organ framework Level of Organization – In animals where organs have related to shape functional frameworks where every framework is related to a particular physiological capacity,  display organ framework level of organization. Examples include Molluscs, Chordates and Annelids etc.

Patterns of Organ Systems

Organs frameworks in various animal groups display different examples of complexities.

• Digestive System – There are two forms of digestive framework Complete and Incomplete Digestive Framework.
• Incomplete Digestive Framework – This form of digestive system has one and only opening to the outside of the body, i.e., a solitary opening serving as both mouth and rear-end. Hence, the digestive system is incomplete.
• Complete Digestive Framework – In this form there are two different openings to the outside of the body, a mouth and a rear-end.
• Circulatory System – Circulatory framework might be of two sorts
• Open Type – In open sort circulatory framework the blood is pumped out of the heart and all the cells and tissues are straightforwardly washed in it. Hence, the circulatory system is open.
• Closed Type – In this kind of circulatory framework, the blood flows through a progression of vessels of varying size and diameters. These include the veins, arteries, and capillaries.

Body Symmetry

We can categorise animals on the basis of the symmetry of their body. The course of action of body parts around a mainline or point decides the symmetry.

• Asymmetrical: We can’t partition these animals into two equivalent parts along with any plane going through their focal point. Example: Sponges.
• Radial Symmetry – Animals tend to display spiral symmetry. This means that any plane going through the focal pivot of the body partitions the organism into two indistinguishable parts. Example: Ctenophores and Echinoderms.
• Bilateral Symmetry – Animals, where the body can be partitioned into indistinguishable right and left parts, are bilaterally symmetrical. Example: Annelids, Arthropods, etc.

Notochord

Chordates are the animals with a notochord. On the other hand, non-chordates are the animals lacking a notochord. Example: Porifera to Echinoderms.

System Classification of Animal Kingdom

Animal Kingdom is classified into:

1. Phylum – Porifera
2. Phylum – Coelenterata (Cnidaria)
3. Phylum – Ctenophora
4. Phylum – Platyhelminthes
5. Phylum – Aschelminthes (Nemotoda) Annelida
6. Phylum – Arthropoda
7. Phylum – Mollusca
8. Phylum – Echinodermata
9. Phylum – Hemichordata
10. Phylum – Chordata

Phylum – Porifera

• Phylum – Porifera includes organisms with holes.
• They are primitive multicellular animals and have cellular level of organisation.
• They are non-motile animals attached to some solid support.
• The body design involves very minimal differentiation and division into tissues.
• They are commonly called sponges.
• They are generally marine and mostly asymmetrical animals.
• Sponges have a water transport or canal system.
• Water enters through minute pores (ostia) in the body wall into a central cavity, spongocoel, from where it goes out through the osculum.
• This pathway of water transport is helpful in food gathering, respiratory exchange and removal of waste.
• The body is supported by a skeleton made up of spicules or spongin fibres.
• Sexes are not separate (hermaphrodite), i.e., eggs and sperms are produced by the same individual.
• Sponges reproduce asexually by fragmentation and sexually by formation of gametes.
• Fertilisation is internal and development is indirect having a larval stage which is morphologically distinct from the adult.

• Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia (Bath sponge).

Phylum – Coelenterata (Cnidaria)

• The name cnidaria is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocytes) present on the tentacles and the body.
• Cnidoblasts are used for anchorage, defense and for the capture of prey.
• Coelenterata (Cnidaria) are aquatic, mostly marine sessile or free-swimming radially symmetrical
• They exhibit tissue level of organization [have more body design differentiation than sponges].
• They have a central gastro-vascular cavity with a single opening.
• They are diploblastic.
• Some of these species live in colonies (corals).
• Some have a solitary [living alone] like–span (hydra).
• Some of the cnidarians, e.g., corals have a skeleton composed of calcium carbonate.
• Cnidarians exhibit two basic body forms called polyp and medusa. The former is a sessile and cylindrical form like Hydra, Adamsia (Sea anemone), etc. whereas, the latter is umbrella-shaped and free-swimming like Aurelia or jelly fish.
• Those cnidarians which exist in both forms exhibit alternation of generation (Metagenesis), i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
• Jellyfish and sea anemones are common examples.
• Digestion is extracellular and intracellular.
• Examples: Aurelia (jelly fish), Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

Phylum – Ctenophora

• Ctenophora are commonly known as sea walnuts or comb jellies.
• They exclusively marine, radially symmetrical, diploblastic
• They exhinit tissue level of organisation.
• The body bears eight external rows of ciliated comb plates, which help in locomotion.
• Digestion is both extracellular and intracellular.
• Bioluminescence (the property of a living organism to emit light) is well-marked in ctenophores.
• Sexes are not separate and reproduction takes place only by sexual means.
• Fertilisation is external [fertilization occurs outside the body] with indirect development [zygote → larvae → animal].
• Examples: Pleurobrachia and Ctenoplana.

Phylum – Hemichordata

• Hemichordata was earlier considered as a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under non-chordata.
• This phylum consists of a small group of worm-like marine animals with organ-system level of organisation.
• They are cylindrical [bilaterally symmetrical], triploblastic, coelomate animals.
• The body is Circulatory system is of open type.
• Respiration takes place through gills.
• Excretory organ is present.
• Sexes are separate. Fertilisation is external. Development is indirect.
• Examples: Balanoglossus and Saccoglossus.

Phylum – Chordata

• Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
• They are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation.
• They possess a post anal tail and a closed circulatory system.
• Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
• Subphyla Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine.
• In Urochordata, notochord is present only in larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life.
• Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Amphioxus or Lancelet.

All chordates possess the following features:

1. have a notochord
2. have a dorsal nerve cord
3. are triploblastic
4. have paired gill pouches
5. are coelomate.

Vertebrata

• These animals have a true vertebral column and internal skeleton, allowing a completely different distribution of muscle attachment points to be used for movement.
• The members of subphylum Vertebrata possess notochord during the embryonic period.
• The notochord is replaced by a cartilaginous or bony vertebral column in the adult.
• Thus all vertebrates are chordates but all chordates are not vertebrates.
• Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation and paired appendages which may be fins or limbs.
• Vertibrates are bilaterally symmetrical, triploblastic, coelomic and segmented, with complex differentiation of body tissues and organs.

Comparison of Chordates and Non-chordates
S.No.	Chordates	Non-chordates
1.	Notochord present.	Notochord absent.
2.	Central nervous system is dorsal, hollow and single.	Central nervous system is ventral, solid and double.
3.	Pharynx perforated by gill slits.	Gill slits are absent.
4.	Heart is ventral.	Heart is dorsal (if present).
5.	A post-anal part (tail) is present.	Post-anal tail is absent.