Animal Kingdom


When you look around, you will observe different animals with different structures and forms. As over a million of species of animals have been described till now, the need for classification becomes all the more important. The classification also helps in assigning a systemic position to newly described species. The basic fundamental features such as level of organisation, symmetry, cell, organisation, coelom, segmentation, notochord etc. have enabled us to broadly classify the animal kingdom. Besides these fundamental features, there are many other distinctive characters, specific for each phylum or class which will be discussed in this chapter.

In Animal Kingdom, which includes all metazoans, first we shall study the charaters of Animal Kingdom. After the study of characters of Animal Kingdom, instead of starting with Phylum Porifera, we will first discuss Phylum Protozoa which is classified infact in Kindom Protista. Whereas, rest of protists will be discussed in Botany classes.


Animjal groups are characterised by mobility, and the presence of a sesory or a nervous system. These systems receive stimuli from the environment and respond by exhibiting some behaviour. The only exception are the profiers (pore-bearers) of the sponges. They have no cell that can be termed as nerve cell. Like plant life, early animals life also arose in sea. The animals which live on the sea floor are called Benthonic (e.g., echinoderms, corals and deep sea fishes), whereas those which swim about actively in sea are called Nektons.

The Multicellular eukaryotic organisms with holozoic mode of nutrition are called metazoans. Basedon complexity of organiosation, metazoans are further sub-divided into two sub-kingdoms, the Parazoa and Eumetazoa.

(a)    Parazoa :      Parazoa includes the sponges in which the cells are loosely aggregated and do not form tissues or organs.
(b)    Eumetazoa          :Eumetazoa includes the rest of the animals in which the cells are organised into structural and funtional units called tissues, orgaqns and organ systems.


1.     Cellular Level : Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of organisations of cells. For example, in sponges, the cells are arranged as loose cell aggregates,
           i.e., they exhibit cellular level of organisation. Some division of labour (activities) occur ammong the cells.

2.    Tissue Level : In coelenterates, the arrangement of cells is more complex. Here the cells performing the same funtion are arranged into tissues, hence is called tissue level of organisation.

3.  Organ Level : A still higher level of organisation, i.e., organ level is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialised for a particular funtion. In animals like Annelids, Arthropods, Molluscs, Echinoderms and Chordates, organs have associated to form funtional systems, each system concerned with a specific physiological funtions. This pattern is called organ system  level of organisations. Organ systems in different groups of animals exhibit various patterns of complexities. For example, the digestive system in Platyhelminthes has only a single opening to the outside of the body that serves as both mouth and anus and is hence called incomplete. A complete digestive system has two openings, mouth and anus Similarly, the circulatory  system may be of two types:
            (i)  Open type
            (ii) Closed type

Blood Vascular System
      Blood vascular system is basically of two types : open and closed.

(i)     Open type :    In this the blood is pumped out of the heart and the cells and tissues are directy bathed in if. The blood is pumped by the heart into the blood cessels that open into blood spaces (sinuses). There is no capillary system (i.e., in most arthropods, non-cephalopod mollusc and tunicates). These sinuses are actually the body cavities, and are called the haemocoel. The pressure of the blood is low; it moves slowly between the tissues, and finally, returns to the heart via the open-ended veins. In fact, distibution of blood to the tissues is very poorly controlled. The pigments, which carry ozygen, remain dissolved in blood plasma. Body tissues and visceral organs exchange repiratory gases, nutrients and waste products, directly with blood.

(ii)    Closed type:    Many invertebrates and all the vertebrates, including humans, have a closes circulatory system. In closed type, the blood flows around the body through the series of blood vessels varying diameter  (arteries, veins and capillaries). In this system, the same blood regularly circulates in the body under high pressure, and returns back to the heart without leaving the system of tubes. The heart pumps the blood into the aorta, which branches in the body into the arteries, and in the tissues into the arterioles, to form the capillary network. The venules of the capillary network carry the blood back to heart via veins and vena cava.This helps in supplying the nutrients and oxygent to the tissues and in removing waste materials and carbon dioxide from it.

      The animals can be catergorised on the basis of their body symmetery:

(i)    Radial Symmetery :   When any plane passing through the central axis of the body divides the organism into two identical halves, it is called as radial symmetry. The animals with radial symmetry are put in the group Radiata. For example, cnidarians (hydra and jelly fish). Biradial symmetry is present in sea anemone.

(ii)     Bilateral Symmetery :    The animals with bilateral symmetry put in groups Bilateria. The body can be divided into identical right and left halves in only one plane. For example, annelids, arthropods.    Asmmetry :   Asymmetric organisms cannot be divided along any plane to produce two equivalent halves. Sponges are mostly asymmetrical.

   Germ layers give rise to all the tissues/organs of the fully formed individuals. On the basis of number of germ layers animals can be

(i)     Diploblastic         : In Diploblastic animals in body cells are arranged in two layers- an outer ectoder and an inner endoderm with an intervening undifferentiated mesoglear. e.g. Coelenterates.

(ii)    Triploblastic        : If the body wall in aniomals is made up of three germ layers i.e. ectoderm, mesoderm and endoderm, they are called triploblastic animals. e.g. Platyhelminthes to chordates.

   Through diverse in shape and size, animals have bodies that fit one of the three basic plans:

(i)     Cell aggregate        : Cell aggregate type of body plan is present in sponges. They are cluseters of cells with a rudimentary division of labout among them. there aqre no tissues no organs.

(ii)     Blind sac               : Blind sac type of body plan is present in  Platyhelminthes and Coelenterates, where the alimentary canal has only one opening.

(iii)   Tube with in Tube    : Tube-whithin tube type of body plan is present in Nemathelmenthes, Annelida, Arthropoda, Mollusca, Echinoderms and Chordates. The digestive system is a continuous tube with an  opening at each end.

   Presence or absense of cavity between the body wall and the gut wall is very important for classification

(i)    Acoelomate        :The animals in which the coelom is absent are called acoelomates, for example poriferans, coelenterates, ctenophora, flatworms. In them the space between ectoderm and endoderm is filled with parenchyma.

(ii)    Pseudocoelomate    :The body cavity is not completely lined with mesoderm. Instead, the mesoderm present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom  e.g. in Roundworm.

(iii)    Eucoelomate        :The true coelom is a body cavity which arises as a cavity in embryonic mesoderm. In this case, the mesoderm of the embryo provides a cellular lining, called coelomic epithelium or peritoneum, to the cavity. The coelom is filled with coelomic fluid secreted by the peritoneum. True coelom is found in  annelids, echinoderms and chordates. True coelom is of two types:

    (a) Schizocoelom    :It develops by the splitting up of mesoderm. It is found in annelids, arthropods and molluscs. Body cavity of arthropods is called haemocoel.
   (b) Enterocoelom    :The mesoderm arises from the wall of the embryonic gut or enteron as hollow outgrowth or enterocoelomic pouches. It occurs in echinoderms and chordates.

Try Yourself
1. Identify the body symmetry that includes an anterior and posterior end.
Ans. Animals with bilateral symmetry have anterior and posterior ends.
2. How is cephalization advatageous to an animal in finding food?
Ans. The head is the site of many sensory receptors and the brain structures that process sensory information. Thus a cephalized animal moving head first can perceive its environment and find food more quickly.


In some animals, the body is externally and internally divided into segments with a serial reptition of at least some organ. For example, in earthworm, the body shows this pattern called metmeric segmentation and the phenomenon is known as metamerism. Metameric segmentation is also present in arthropods, annelids and chordates.


Notochord is mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. Animals with notochord are called chordates and those aniomals which do not have this structure are clalled non-chordates, e.g., porifera to echinoderms.

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