● There are a millions of different animals around us with difference in their structure and form.



●In spite of these differences, there are many fundamental features common to various individuals, which are:

`star` Levels of organization of cells (Pattern of digestive, circulatory or reproductive systems within organ system organisation)
`star` Body Plan
`star` Body symmetry
`star` Nature of coelom
`star` Pattern of embryonic layers.
`star` Segmentation of the body
`star` Presence or absence of Notochord

●Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of organisation of cells. There are different levels of organisation.

`color{Brown}"Cellular level of organisation"` : E.g. sponges

● The cells are arranged as `color{Violet}"loose cell aggregates"`.
● Some division of labour (activities) occur among the cells. In coelenterates, the arrangement of cells is more complex.

`color{Brown}"Tissue level of organisation"`: E.g. Coelentrates and Ctenophores
● Here the cells performing the same function are arranged into tissues.

`color{Brown}"Organ level of organisation"`: E.g. Members of Platyhelminthes and other higher phyla.
● Here the tissues are grouped together to form organs, each specialized for a particular function.

`color{Brown}"Organ system level of organisation"`: E.g. animals like Annelids, Arthropods, Molluscs,

●Echinoderms and Chordates Here organs have associated to form functional systems, each system concerned with a specific physiological function.

● Organ systems in different groups of animals exhibit various patterns of complexities.

● The `color{Brown}"Digestive System"`: Maybe two types:

`star` `color{Violet}"lncomplete"`: 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.

`star` A `color{Violet}"Complete"` digestive system has two openings, mouth and anus.

● The `color{Brown}"circulatory system"`: May be of two types:

`star` `color{Violet}"Open type"`: In which the blood is pumped out of the heart and the cells and tissues are
directly bathed in it

`star` `color{Violet}"Closed type"`: in which the blood is circulated through a series of vessels of varying diameters
(arteries, veins and capillaries).

Animals show various body plans

● `color{Brown}"Cell Aggregate type"`: E.g. Sponges

● `color{Brown}"Blind Sac type"`: In them the digestive system is incomplete; it just has a single opening to the outside of the body that serves as both the mouth and the anus. E.g.: Coelenterates to Platyhelminthes

● `color{Brown}"Tube within tube type"` : Found in animals with complete digestive tracts i.e. with separate openings of mouth and anus. E.g.: Nemathelminthes to Chordates

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

`color{Brown}"Asymmetrical "`: Sponges are mostly asymmetrical, i.e., any plane that passes through the centre does not divide them into equal halves.

`color{Brown}"Radial symmetry"`: When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry. E.g. Coelenterates, Ctenophores and Echinoderms

`color{Brown}"Bilateral symmetry"`: Animals like Annelids, Arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry.

`color{Brown}"Diploblastic animals"`: Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., Coelenterates and Ctenophora. An undifferentiated jelly like layer, mesoglea, is present in between the ectoderm and the endoderm.

`color{Brown}"Triploblastic animals"`: Those animals in which the developing embryo (gastrula) has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals E.g. Platyhelminthes to Chordates.

Coelom: The body cavity, which is lined by mesoderm

● Presence or absence of a cavity between the body wall and the gut wall is very important in classification.

● The body cavity, which is lined by mesoderm is called coelom.

`color{Brown}"Coelomates"`: Animals possessing coelom are called coelomates, e.g.,Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates and Chordates

`color{Brown}"Pseudocoelomates"`: In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm.
Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates, e.g., Aschelminthes

`color{Brown}"Acoelomates"`: The animals in which the body cavity is absent are called acoelomates, e.g., Platyhelminthes.

● In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs. The various types of segmentations are:

● `color{Brown}"Unsegmented"`: Body is unsegmented in Poriferans, Coelenterates and Aschelminthes.

● `color{Brown}"Psudometameric segmentation"`: Segmentation is called pseudometameric, when each segment is almost independent and the new segments are added from the anterior end. e.g., Tapeworm

● `color{Brown}"Metameric segmentation"`: Segmentation is called metameric, when the segments are dependent on each other and new segments are added from the posterior end. E.g.: Annelids, Arthropods and Chordates.

Notochord: A mesodermally derived (tubular) rod-like structure formed on the dorsal side (between the nerve cord and alimentary canal); during embryonic development in some animals.

● `color{Brown}"Chordates"`: Animals with notochord in some stage of their life are called chordates

● `color{Brown}"Non-chordates"`: Those animals which do not form this structure at any stage are called non-chordates, e.g., Porifera to Echinoderms.