This document provides an overview of the key characteristics used to classify members of the animal kingdom, including their: 1) Level of organization (cellular to organ system); 2) Symmetry (asymmetrical, radial, bilateral); 3) Embryonic germ layers (diploblastic, triploblastic); 4) Presence of coelom or pseudocoelom; 5) Segmentation and metamerism in some animals; 6) Presence or absence of a notochord in chordates. The types of digestive and circulatory systems are also discussed.

1. ANIMAL
KINGDOM-INTRODUCTION

2. GENERAL CHARACTERS OF MEMBERS OF ANIMAL KINGDOM
(a) Multicellular
(b) Heterotrophic
(c) Lack cell wall
(d) Eukaryotic
The members of animal kingdom are divided into various phyla and
classes based on certain criteria such as
1) Level of organization
2) Symmetry
3) Whether diploblastic/triploblastic.
4) Nature of coelom.
5) Segmentation of the body
6) Presence/absence of Notochord
7) Type of circulatory system / Digestive system.

3. Levels of Organisation
Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of
organisation of cells.
(A)CELLULAR LEVEL
In sponges, the cells are arranged as loose cell aggregates, i.e., they exhibit cellular
level of organisation. Some division of labour (activities) occur among the cells.
(B) TISSUE LEVEL
In coelenterates like Hydra, the arrangement of cells is more complex. Here
the cells performing the same function are arranged into tissues, hence is
called tissue level of organisation.
(C) ORGAN LEVEL
A still higher level of organisation, i.e., organ level is exhibited by members of Platyhelminthes(tape
worm,Planaria etc.) tissues are grouped together to form organs, each specialized for a particular
function.
(D) ORGAN SYSTEM LEVEL
In animals like Annelids, Arthropods, Molluscs, Echinoderms and Chordates, organs have
associated to form functional systems, each system concerned with a specific physiological
function. This pattern is called organ system level of organisation.

4. Symmetry
Animals can be categorised on the basis of their
symmetry.
(A) ASYMMETRICAL
These animals cannot be divided into 2 equal halves
through any plane passing through their centre.
Eg-Phylum Porifera-sponges.
(B) RADIALLY SYMMETRICAL
When an organism can be cut into two identical
halves in more than one plane passing through the
centre it is called radial symmetry.
Eg- Members of Coelenterata, Ctenophora and
Echinodermata.
(C) BILATERALLY SYMMETRICAL
These animals can be divided into 2 equal halves
through only one plane passing through the
centre.
Eg Annelids, Arthropods, Chordates etc.

5. Diploblastic and Triploblastic
Organisation
DIPLOBLASTIC
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. An undifferentiated
(non-cellular) layer mesoglea, is present in
between the ectoderm and the endoderm.
TRIPLOBLASTIC
The embryo of these animals have 3 germ
layers-outer ectoderm,middle mesoderm and
inner endoderm.
Eg- Platyhelminthes to Chordates.

7. NATURE OF COELOM
Presence or absence of a cavity between the body
wall and the gut wall (wall of alimentary canal) is very important
in classification.
The body cavity, which is lined by mesoderm on both sides is
called true coelom.
ACOELOMATE ANIMALS
The animals in which the body cavity is absent are called
acoelomates, e.g., Platyhelminthes
PSEUDOCOELOMATE ANIMALS
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
COELOMATE ANIMALS
Animals possessing coelom are called coelomates, e.g.,
Annelids, Molluscs, Arthropods, Echinoderms,
Hemichordates and Chordates

8. Segmentation
In some animals, the body is externally and internally divided into segments with a
serial repetition of at least some organs. For example, in earthworm, the body shows
this pattern called metameric segmentation and the phenomenon is known as
metamerism.

9. Notochord
Notochord is a
mesodermally derived
rod-like structure formed
on the dorsal side during
embryonic development in
some animals. Animals
with notochord are called
chordates and those
animals which do not
form this
structure are called
non-chordates, e.g.,
Porifera to Echinoderms
CHORDATE-CHARACTERS

10. Digestive system
INCOMPLETE
In Platyhelminthes the digestive canal has only a
single opening to the outside of the body that serves as
both mouth and anus, and is hence called incomplete.
COMPLETE DIGESTIVE SYSTEM
A complete digestive system has two openings, mouth
and anus. Eg-Annelida to Chordates.
CIRCULATORY SYSTEM
The circulatory system may be of two types:
(i) open type in which the blood is pumped
out of the heart and the cells and tissues are
directly bathed in it Eg-Arthropoda.
(ii) closed type in which the blood is circulated
through a series of vessels of varying diameters
(arteries, veins and capillaries).Blood never flows out of
the blood vessels.

11. INCOMPLETE DIGESTIVE SYSTEM IN PLANARIA

12. COMPLETE DIGESTIVE SYSTEMS-WITH
SEPARATE MOUTH AND ANAL OPENING.