ANIMAL KINGDOM : BASIS OF CLASSIFICATION

 Animals are heterotrophic, multicellular organisms devoid of chlorophyll and cell walls.
Animalia has eleven main phyla: For detail notes

1. Porifera 7. Invertebrates
 2. Cnideria 8. Mollusca
 3. Ctenophora 9. The echinodermata
 4. Hemichordata 10. Platyhelminthes
 5. Aschelminthes 11. Chordata
 6. Annelida

 CLASSIFICATION BASIS

1. Organizational levels

 
 Animals are categorized into four levels based on this:
a.At the cellular level of organization, cells are grouped together loosely. For instance, Porifera.

b. The organizing of cells into tissues occurs at the tissue level. For instance, ctenophores and nidarians.
 

c. Organ level organization: Tissues are grouped into organs at this level. For example, higher animals (from chordates to platyhelminthes).
 

 d. Organ system level of organization: In this case, an organ system is linked to an organ. Every system carries out a particular physiological task. Consider higher animals.
 Animal organ systems demonstrate

2. body symmetry

 

Identical bodily parts are arranged on either side of the body's central axis. There are two categories of creatures based on symmetry: asymmetrical and symmetrical.
 

a. Asymmetrical: The body is unable to be split into two equal parts. Like most poriferans, snails, etc.

 

b. Symmetrical: The body can be split into two equal parts in this case. There are two kinds.

 • Radial symmetry: In this case, the body's central axis (oral-aboral axis) can be divided into two equal halves in any vertical plane. For instance, certain cnidarians, echinoderms (adult), ctenophores, and poriferans.

 • Bilateral symmetry: The body can be split into the left and right halves in a single plane in this instance.

For instance, Platyhelminthes to Chordata (apart from mature Echinodermata).

An animal that is bilaterally symmetrical has an anterior (cephalic) side, a posterior (anal or tail) side, a ventral side (lower), left and right lateral sides, and an anterior (dorsal) side.
 

3. Embryonic layers, or germinal layers  

All of the body's organs develop from these layers of the embryo. There are two sorts of animals: diploblastic and triploblastic, based on the number of germ layers.

A. diploblastic animal consists of two germ layers: the inner endoderm and the outer ectoderm. Mesoglea, an undifferentiated layer that resembles jelly, is found in between these layers. Cnidaria & Ctenophora, for instance.

 

b. Triloblastic animals: outer ectoderm, middle mesoderm, and inner endoderm are the three germ layers. Platyhelminthes to Chordata, for instance

 

4.Coelom (body cavity)

It is the mesoderm-lined cavity. It is seen between the gut wall and the body wall. The coelom divides the abdominal and wall muscles.
Animals can be divided into three categories based on their coelom:

a. Acoelomate:  Devoid of coelom. Matrix (parenchyma) fills the gap between the stomach cavity and the body wall.

 Porifera to Platyhelminthes, for example. 

b. False coelom, or pseudoelomate. Mesoderm does not line the body cavity in this instance. Pockmarks are dispersed in mesoderm.

 For instance, Aschelminthes.

c. True coelom: Coelomate. Here, the mesoderm gives rise to the coelom. The peritoneal layer lines the coelom, which is then filled with coelomic fluid. For instance, Annelida to Chordata. 

 
 

The coelomic fluid works as a shock absorber and decreases friction between visceral organs, while the coelom accommodates the organs.
 

5.Metamerization (division)

 It is the phenomena where the body or its organs are split into recurring sections both inside and outside (metameres). Such as arthropods and annelids (earthworms, etc.).
 

6. The Notochord

 In certain animals during embryonic development, it is a mesodermally derived supporting rod that forms on the dorsal side.
Animals that lack a notochord are referred to as non-chordates, and those that have are known as chordates.