CLASSIFICATION OF ANIMALS CONTINUATION BY VINU.
M(MV)
Department of Biology
Classification of animals based on Symmetry:
Symmetry in biology refers to the correspondence of the body parts, in size,
shape and relative position, on opposite sides of a dividing line or distributed
around a central point or axis. On the basis of their symmetry animals are
broadly categorized into 3 types namely:
1. Asymmetrical: This includes animals when divided in any plane passing
through the centre does not divide the body into equal halves. Ex:
Porifera(sponges).
2. Radially symmetrical: This includes animals when divided along any plane
passing through the central axis of the body divides the animal into two
equal halves. Ex: Coelenterates, Ctenophores and Echinodermata.
3. Bilaterally symmetrical: This includes animals when divided in only one
longitudinal or vertical plane into identical or similar left and right halves. Ex:
Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Hemichordata
and Chordata.
DIPLOBLASTIC AND TRIPLOBLASTIC ORGANISATION:
Animals are broadly classified into two types based on the number of
embryonic or germ layers(the cluster of cells formed during embryonic
stage gastrula that are arranged into 2 or 3 layers from which various
tissues and organs arise):
1. Diploblastic animals: This includes animals in which the cells are
arranged into two embryonic/ germ layers namely an outer ectoderm
and an inner endoderm with an undifferentiated jelly like mesoglea in
between the ectoderm and endoderm. Ex: Coelenterata and
Ctenophora
2. Triploblastic animals: This includes animals in which the cells are
arranged into three embryonic/germ layers namely an outer ectoderm,
middle mesoderm and inner endoderm. Ex: Platyhelminthes to
Chordata
Classification of animals based on the presence or absence of Coelom:
Coelom is the inner body space or body cavity which is present between the
outer body wall and the inner alimentary canal. All the organ systems, tissues
and organs will be present in the body cavity or coelom. Animals are broadly
divided into three types based on the presence or absence of coelom namely:
1. Coelomates: They are the animals that possess a true coelom or body cavity
which is lined by mesoderm. E.g., Annelids, Arthropoda, Mollusca,
Echinodermata, Hemichordata and Chordata.
2. Pseudocoelomates: They are the animals that have a false body cavity not
lined by mesoderm and the mesoderm occurs as scattered pouches in
between the ectoderm and endoderm. Such a body cavity is called
pseudocoelom. E.g., Aschelminthes.
3. Acoelomates: They are the animals in which the body cavity is absent. E.g.,
Platyhelminthes.
Classification of animals based on segmentation:
Segmentation is the process of division of the body both internally and externally
into equal or similar ring like parts called segments. Each segment consists of a
set of all the organ systems which is serially repeated. Such a type of
segmentation is called metameric segmentation and the phenomenon is called
metamerism. E.g., Earthworm.
False segmentation or pseudometamerism is the process of division of the body
only externally into unequal ring like parts called proglottids. E.g., Tapeworm.
Unsegmented: In some animals the body is not divided into equal ring like parts.
E.g., Hydra, Liver fluke, Star fish.
Classification of animals based on the presence or absence of notochord:
Notochord is a solid skeletal supporting rod-like structure, derived from the
mesoderm which is formed on the dorsal side of the body during embryonic
development in some animals. Based on the presence or absence of notochord
Animals are broadly divided into :
1. Chordata or vertebrata: Animals in which notochord or vertebral column is
present like urochordates, fishes, frogs, reptiles, aves and mammals.
2. Non-chordata or invertebrate: Animals in which notochord is not formed or
absent. E.g., Porifera to Hemichordata.
TAPEWORM
PLANARIA EARTHWORM
