Basis of classification for Animals

• Animals are classified and placed on an appropriate group on the basis of observable characters. Some of the characters to classify the organism are being described below:

Levels of Organisation

• All members of Animals  are multicellular but  all of them do not have  same pattern of organisation of cells. 

• For example,  In sponges. the cells are arranged as loose aggregation and every cell perform diffrent function called as cellular level of organisation. The  division of labour is present among the cells. 

• In coelenterates, the arrangement of cells is more complex. The cells  are arranged Into tissues to perform the same function.This  is called tissue level of organisation. 

• A another  level of organisation called organ level has been first developed by members of Platyhelminthes. In this level of organisation, tissues are arranged in  form of  organs. Each specialised organ perform  a particular function. 

• ln animals like Annelids. Arthropods. Molluscs.Echinoderms and Chordates. Number of organs have been organised in a functional systems. Each system is  associated  with  the specific physiological function. This pattern ls called organ system level of organisation. 

Digestive system  

• The digestive system in which there is a single opening to the outside of the body that serves as both mouth and anus. This  type of digestive system is incomplete. 

• A complete digestive system has two openings to serve as  mouth and anus separately. 

Circulatory system

• In open type of Circulatory system,  the blood Is pumped out of the heart. The cells and tissues are directly bathed in blood.

• In closed type of Circulatory system, the blood is circulated through a network  of  blood vessels  like artertes. veins and capillary.

Symmetry

• Animals can be categorised on the basis of their symmetry. 

• When , any plane is passed through the centre of body and it does not divide into equal halves than body is said to be asymmetrical. For Example - Poriferan or Songes.

• When any plane is passed through the central axis of the body and it is  divided  the organism Into two identical halves. This one is said to be radial symmetrical . For Example Coelenterates. Ctenophora

• Animals like annelids. arthropods. etc.. where the body can be divided into Identical left and right halves in only one plane. This is a  bilateral symmetrical.

Diploblastic and Triploblastic Organisation

• During embryonic development, the animals  which have the  two embryonic layers. an external ectoderm and an internal endoderm. are called diploblastic animals. e.g.coelenterates. 

• An undifferentiated  layer mesoglea is present in between the ectoderm and the endoderm in some Coelenterates.

• Those animals In which the developing embryo has a three germinal layer : outer ectoderm, middle  mesoderm and inner endoderm. Such  are called triploblastic animals For example : Platyhelminthes to chordates. 



Coelom

• Presence or absence of a cavity between the body wall and the gut wall is very important feature  in classification. The body cavity which ls lined by mesoderm is called coelom . 

• Animals with coelom are called coelomates, e.g.. annelids. molluscs. arthropods. echinoderms. hemichordates and chordates 

• In some animals. the body cavity is not lined by mesoderm but mesoderm is present as scattered pouches In between the ectoderm and endoderm. Such a body cavity ls called pseudocoelom and the animals with  pseudocoelom are called  pseudocoelomates. For example:  Aschelmlnthes.

• The animals In which the body cavity is absent are called acoelomates. For example  platyhelminthes .

Segmentation

• In some animals, the body is externally and internally divided Into segments. 

• But in earthworm,  the body is   metamerically  segmented in which external segments make correspondence to internal segments. This phenomenon is known as metamerism .

Notochord

• Notochord is  derived from the mesoderm. Notochord is  rod- like structure present  on  dorsal side during embryonic development in some animals. 

• Animals with  notochord are called chordates and those animals which do not have notochord  are called non-chordates, For example : Porifera to Echinoderms.

• In some choradates, Notochord is replaced by cartilaginous or bony vertebral column. Such choradates are called Vertebrates .

• In some choradates, notochord is present as notochord in any stage of lifecycle but not replaced by  vertebral column. They are not vertebrates.

• Therefore all vertebrates are choradates but all choradates are not vertebrates.