Classification of Animal Kingdom

The largest kingdom in the classification consists of various kinds of animals. The nutritional mode of every organism is different. RH Whittaker classified the organisms on different basis like on the basis of nutritional mode, their cell structure etc. Nowadays, a lot of new species are getting discovered, and it is getting more and more complex to classify the organisms. So, there is a need for common criteria through which classification of species becomes easy. The common features among species will be a way to categorize them into different, as follows.

Organization level of organisms 

In this category, the organisms are divided on the basis of the level of functioning they have in their systems as explained as follows

1. Cellular level – This category is for those animals in which cells are arranged as loose aggregates. For e.g. Sponges.
2. Tissue level – Organisms in which cells perform the same function get arranged as tissues fall under this category. For e.g. Coelenterates.
3. Organ level – In many organisms, there is an organ level of organizations, which means that tissues performing the same activities are grouped together to form an organ. Each organ has its specific function. For e.g. Platyhelminthes.
4. Organ system level – This type of organization is seen more commonly in many species. For e.g., Arthropods, Echinoderms, Chordates etc. represent that organs form a group to perform a functioning system where every system (group of organs) is assigned to perform a certain physiological function.

Symmetry 

Symmetry in animals represents the similar parts or proportions formed after the division of the organism by any plane through its axis. On the basis of the thin, the animal kingdom is divided into three parts.

1. Asymmetrical – The organism which shows no symmetry after division through any axis are called asymmetrical organisms. For e.g. Sponges.
2. Radial Symmetry – The organism which divides into two exactly equal parts when any plane passes through the axis. Such organisms are called radially symmetrical. For e.g. Ctenophores, Coelenterates etc.
3. Bilateral Symmetry – Only one plane divides the body into two exact, equal halves. Such species are known as bilaterally symmetrical. For e.g. Molluscs, Arthropods etc.

Germ layers 

 At the blastula stage of the organism, the presence of the number of germ layers defines whether the organism is diploblastic or triploblastic.

1. Diploblastic organisms – If an organism is having two germ layers in the adult stage, i.e. ectoderm and endoderm. Such organisms are called diploblastic organisms. For e.g. Cnidarians and Ctenophores.
2. Triploblastic organisms – As an adult, the organism is having three embryonic germ layers i.e. ectoderm, mesoderm and endoderm. Such organisms are called triploblastic organisms. For e.g. Platyhelminthes, Chordates etc.

Segmentation 

The organisms which have particular patterns on their body or segments come under this category. For e.g. Arthropods, Chordates, Annelids etc.

 Coelom 

The main cavity in the body of an organism is called a coelom. Mesoderm lined the coelom. On the basis of the presence or absence of coelom, organisms are divided into three categories as follows.

1. Acoelomate – Organisms in which coelom or cavity is absent are known as acoelomates. For e.g. Platyhelminthes.
2. Coelomate – Organisms having proper cavity or coelom are called coelomate. For e.g. Annelids, Molluscs, Arthropods etc.
3. Pseudocoelomates – These organisms have pseudo coelom i.e. they don’t have a proper cavity, and it’s often known as a false cavity. They have patches of mesoderm, so the cavity is not lined by mesoderm. For e.g. Aschelminthes.

 Notochord 

Rod-like structure originated from mesoderm and provides support to organisms in the embryonic stage and as adults. Organisms are characterized by the presence and absence of notochord.

1. Chordata -Organisms having notochord are known as chordates. For e.g. Vertebrates, mammals, tunicates etc.
2. Non–Chordata – Organisms to lack notochord are called non-chordates. For e.g. Platyhelminthes, Ctenophores, Annelids etc.

Conceptual Questions

Question 1: What are the advantages of the basis of classification.

Answer:

Through the basis of classification, study of organisms has become easier. It’s get easier to learn about organisms and compare the one group of organism to another  for better understanding.

Question 2: Find the odd one out and give the reason why – Annelids, Molluscs, Aschelminthes, Echinoderms

Answer:

Aschelminthes,- all others are coelomates and aschelminthes are pseudocoelomates.

Question 3: Separate out into diploblastic and triploblastic organisms – Platyhelminthes, Cnidarians, Chordates, Ctenophores.

Answer:

Diploblastic organisms-Cnidarians, Ctenophores. 

Triploblastic organisms- Platyhelminthes, Chordates.

Question 4: Explain the difference between Amphibia and Reptilia on the basis of their skin.

Answer: 

Amphibians have moist and slimy skin and they lay eggs in water whereas Reptilians have dry skin covered with scales and they do not lay eggs in water.

Question 5: Explain metamerism.

Answer: 

In some organisms, there are repetitions of body parts and have many segments on their body. Such orientation of body segmentation is called metamerism. For eg. Earthworm 

Question 6: What do you understand by protochordate? 

Answer: 

Lower chordates have some organisms which are found in marine water. They have a coelom, and bilateral symmetry and are triploblastic. They don’t have a notochord from the start but at a stage they develop a notochord. Such organisms are called protochordate. For eg. Hemichordates, Cephalochordata.

Question 7: Explain the importance of coelom in animals.

Answer: 

The cavity between the body wall and the alimentary canal of organisms is called coelom. Mesoderm layer lines coelom. The organs which are not in use ( visceral organs) lie in the coelom. 

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