Textbook: Animal Classification | Science and Technology Class 10 (Maharashtra SSC Board) PDF Download

 Page 1


61
Which criteria are used for classification of organisms?
6. Animal Classification
Ø History of animal classification
Ø New (method) system of animal classification.
Ø Kingdom- Animalia.
 You have studied the classification of living organisms in earlier classes. The living 
organisms present around us are mainly plants and animals. We have studied the criteria 
of their classification. With the help of that, complete the following chart.  
  Kingdom: Fungi 
 Kingdom: Plantae 
  Kingdom: Animalia
Kingdom: Monera
Multicellular
 organisms
Unicellular 
organisms
Kingdom: -----------
Living organisms
 How the plants are classified?
  We have studied the plants classification in last year. It helped us to understand the 
diversity of plants around us.
 You must be looking varieties of animals around you. Some animals are too small 
whereas some are too big. Some animals are terrestrial where as some are aquatic. Some 
animals crawl on land, some swim in water whereas some fly in air. Some animals have 
scales on skin whereas some have feathers or hairs. In this way, there is huge diversity 
among the animals too. According to recent studies, estimated number of animal species 
on earth is approximately 7 million. It is impossible to study each and every species. 
However, if groups and sub-groups of animals are formed depending upon the similarities 
& differences, it will make it very easy to study such vast variety of animals.
6.1  Classification of living organisms
Can you recall?
Can you recall?
Page 2


61
Which criteria are used for classification of organisms?
6. Animal Classification
Ø History of animal classification
Ø New (method) system of animal classification.
Ø Kingdom- Animalia.
 You have studied the classification of living organisms in earlier classes. The living 
organisms present around us are mainly plants and animals. We have studied the criteria 
of their classification. With the help of that, complete the following chart.  
  Kingdom: Fungi 
 Kingdom: Plantae 
  Kingdom: Animalia
Kingdom: Monera
Multicellular
 organisms
Unicellular 
organisms
Kingdom: -----------
Living organisms
 How the plants are classified?
  We have studied the plants classification in last year. It helped us to understand the 
diversity of plants around us.
 You must be looking varieties of animals around you. Some animals are too small 
whereas some are too big. Some animals are terrestrial where as some are aquatic. Some 
animals crawl on land, some swim in water whereas some fly in air. Some animals have 
scales on skin whereas some have feathers or hairs. In this way, there is huge diversity 
among the animals too. According to recent studies, estimated number of animal species 
on earth is approximately 7 million. It is impossible to study each and every species. 
However, if groups and sub-groups of animals are formed depending upon the similarities 
& differences, it will make it very easy to study such vast variety of animals.
6.1  Classification of living organisms
Can you recall?
Can you recall?
62
 Formation of groups and sub-groups of animals depending upon similarities and 
differences among animals is called as animal classification. 
Benefits of animal classification
1. Study of animals becomes 
convenient.
2. Study of few animals from a 
group helps to understand about 
that entire animal group.
3. It gives idea about animal 
evolution.
4. Animals can be easily identified 
with great accuracy.
5. It helps to understand the 
relationship of animals with 
other living organisms.
6. It helps to understand the 
habitat of each animal and it's 
exact role in the nature.
7. It helps to understand various 
adaptations shown by animals.
 Time to time, different scientists have tried 
to classify the animals. Greek philosopher 
Aristotle was the first to perform the animal 
classification. Aristotle classified the animals 
according to the criteria like body size, 
habits and habitats. Further, as per the new 
developments in sciences, references were 
changed and thereby the criteria of animal 
classification too. Classification proposed 
by Aristotle is known as ‘Artificial method’. 
Besides Aristotle, artificial method of 
classification was followed by Theophrastus, 
Pliny, John Ray, Linnaeus, etc. Later on, 
‘Natural system of classification’ was followed. 
Natural system of classification was based on 
various criteria like body organization, types of 
cells, chromosomes, bio-chemical properties, 
etc. By the time, system of classification based 
on evolution was also brought into practice. It 
was used by Dobzhansky and Meyer. Recently, 
Carl Woese has also proposed the animal 
classification
 Traditionally, depending upon presence or absence of the notochord, the animal 
kingdom has been divided into two groups- Non-chordates and Chordates.
History of animal classification
 Non-chordates are classified / divided into ten phyla. Those phyla are- Protozoa, 
Porifera, Coelentarata / Cnidaria, Platyhelminthes, Aschelminthes, Annelida, 
Arthropoda, Mollusca, Echinodermata and Hemichordata
Traditional method of animal classification
A. Non-Chordates : Characters of non-chordate animals are as follows
1. Body is not supported by rod-like notochord.
2. Pharyngeal gill-slits are absent.
3. Nerve cord; if present, it is on ventral side. It is solid & paired.
4. Heart, if present, it is on dorsal side.
Page 3


61
Which criteria are used for classification of organisms?
6. Animal Classification
Ø History of animal classification
Ø New (method) system of animal classification.
Ø Kingdom- Animalia.
 You have studied the classification of living organisms in earlier classes. The living 
organisms present around us are mainly plants and animals. We have studied the criteria 
of their classification. With the help of that, complete the following chart.  
  Kingdom: Fungi 
 Kingdom: Plantae 
  Kingdom: Animalia
Kingdom: Monera
Multicellular
 organisms
Unicellular 
organisms
Kingdom: -----------
Living organisms
 How the plants are classified?
  We have studied the plants classification in last year. It helped us to understand the 
diversity of plants around us.
 You must be looking varieties of animals around you. Some animals are too small 
whereas some are too big. Some animals are terrestrial where as some are aquatic. Some 
animals crawl on land, some swim in water whereas some fly in air. Some animals have 
scales on skin whereas some have feathers or hairs. In this way, there is huge diversity 
among the animals too. According to recent studies, estimated number of animal species 
on earth is approximately 7 million. It is impossible to study each and every species. 
However, if groups and sub-groups of animals are formed depending upon the similarities 
& differences, it will make it very easy to study such vast variety of animals.
6.1  Classification of living organisms
Can you recall?
Can you recall?
62
 Formation of groups and sub-groups of animals depending upon similarities and 
differences among animals is called as animal classification. 
Benefits of animal classification
1. Study of animals becomes 
convenient.
2. Study of few animals from a 
group helps to understand about 
that entire animal group.
3. It gives idea about animal 
evolution.
4. Animals can be easily identified 
with great accuracy.
5. It helps to understand the 
relationship of animals with 
other living organisms.
6. It helps to understand the 
habitat of each animal and it's 
exact role in the nature.
7. It helps to understand various 
adaptations shown by animals.
 Time to time, different scientists have tried 
to classify the animals. Greek philosopher 
Aristotle was the first to perform the animal 
classification. Aristotle classified the animals 
according to the criteria like body size, 
habits and habitats. Further, as per the new 
developments in sciences, references were 
changed and thereby the criteria of animal 
classification too. Classification proposed 
by Aristotle is known as ‘Artificial method’. 
Besides Aristotle, artificial method of 
classification was followed by Theophrastus, 
Pliny, John Ray, Linnaeus, etc. Later on, 
‘Natural system of classification’ was followed. 
Natural system of classification was based on 
various criteria like body organization, types of 
cells, chromosomes, bio-chemical properties, 
etc. By the time, system of classification based 
on evolution was also brought into practice. It 
was used by Dobzhansky and Meyer. Recently, 
Carl Woese has also proposed the animal 
classification
 Traditionally, depending upon presence or absence of the notochord, the animal 
kingdom has been divided into two groups- Non-chordates and Chordates.
History of animal classification
 Non-chordates are classified / divided into ten phyla. Those phyla are- Protozoa, 
Porifera, Coelentarata / Cnidaria, Platyhelminthes, Aschelminthes, Annelida, 
Arthropoda, Mollusca, Echinodermata and Hemichordata
Traditional method of animal classification
A. Non-Chordates : Characters of non-chordate animals are as follows
1. Body is not supported by rod-like notochord.
2. Pharyngeal gill-slits are absent.
3. Nerve cord; if present, it is on ventral side. It is solid & paired.
4. Heart, if present, it is on dorsal side.
63
 This system of animal classification was in practice till now. However, now a days, new 
system of classification is followed. We will study this new system of animal classification 
in brief.
 At present, according to the five kingdom classification system of Robert Whittaker, all 
multicellular animals are included in Kingdom: Animalia. This system of classification is 
based upon some criteria like Body organization, Body symmetry, Body cavity, Germinal 
layers, Segmentation, etc.
 All chordate animals are grouped together in a single phylum and the name of 
phylum is same i.e. Phylum- Chordata. This phylum has been divided into three 
subphyla as- Urochordata, Cephalochordata & V ertebrata. Sub-phylum V ertebrata has 
been further divided into six classes as- Class: Cyclostomata, Class: Pisces, Class: 
Amphibia, Class: Reptilia, Class: Aves and Class: Mammalia.
Do you know?
1.Cyclostomata
2. Pisces
3. Amphibia
4. Reptilia
5. Aves
6. Mammalia 
 
1. Protozoa
2. Porifera
3. Cnidaria
4.Platyhelminthes
5.Aschelminthes
6. Annelida
7. Arthropoda
8. Mollusca
9. Echinodermata 
10. Hemichordata  
1. Urochordata 
Chordates
Non-
chordates
Kingdom:
Animalia
3. Vertebrata
2. Cephalochordata
Phylum-
Chordata
Subphylum
Class  
6.3 Conventional System of  Animal Classification
Chordates : Characters of chordates are as follows
1. Body is supported by notochord.
2. Pharyngeal gill-slits or lungs are present for respiration.
3. Nerve cord is present on dorsal side of body. It is hollow. 
4. Heart is present on ventral side of body.
Phylum
 Notochord is a long rod like 
supporting structure present on 
dorsal side of animal body. It 
keeps the nerve tissue isolated from 
remaining body.
Sub Kingdom
Sub 
Kingdom:
Anus
Tail fin
Muscle
6.2 Characteristics of Chordates  
Notochord
Nerve chord
Mouth 
Gill slits 
Page 4


61
Which criteria are used for classification of organisms?
6. Animal Classification
Ø History of animal classification
Ø New (method) system of animal classification.
Ø Kingdom- Animalia.
 You have studied the classification of living organisms in earlier classes. The living 
organisms present around us are mainly plants and animals. We have studied the criteria 
of their classification. With the help of that, complete the following chart.  
  Kingdom: Fungi 
 Kingdom: Plantae 
  Kingdom: Animalia
Kingdom: Monera
Multicellular
 organisms
Unicellular 
organisms
Kingdom: -----------
Living organisms
 How the plants are classified?
  We have studied the plants classification in last year. It helped us to understand the 
diversity of plants around us.
 You must be looking varieties of animals around you. Some animals are too small 
whereas some are too big. Some animals are terrestrial where as some are aquatic. Some 
animals crawl on land, some swim in water whereas some fly in air. Some animals have 
scales on skin whereas some have feathers or hairs. In this way, there is huge diversity 
among the animals too. According to recent studies, estimated number of animal species 
on earth is approximately 7 million. It is impossible to study each and every species. 
However, if groups and sub-groups of animals are formed depending upon the similarities 
& differences, it will make it very easy to study such vast variety of animals.
6.1  Classification of living organisms
Can you recall?
Can you recall?
62
 Formation of groups and sub-groups of animals depending upon similarities and 
differences among animals is called as animal classification. 
Benefits of animal classification
1. Study of animals becomes 
convenient.
2. Study of few animals from a 
group helps to understand about 
that entire animal group.
3. It gives idea about animal 
evolution.
4. Animals can be easily identified 
with great accuracy.
5. It helps to understand the 
relationship of animals with 
other living organisms.
6. It helps to understand the 
habitat of each animal and it's 
exact role in the nature.
7. It helps to understand various 
adaptations shown by animals.
 Time to time, different scientists have tried 
to classify the animals. Greek philosopher 
Aristotle was the first to perform the animal 
classification. Aristotle classified the animals 
according to the criteria like body size, 
habits and habitats. Further, as per the new 
developments in sciences, references were 
changed and thereby the criteria of animal 
classification too. Classification proposed 
by Aristotle is known as ‘Artificial method’. 
Besides Aristotle, artificial method of 
classification was followed by Theophrastus, 
Pliny, John Ray, Linnaeus, etc. Later on, 
‘Natural system of classification’ was followed. 
Natural system of classification was based on 
various criteria like body organization, types of 
cells, chromosomes, bio-chemical properties, 
etc. By the time, system of classification based 
on evolution was also brought into practice. It 
was used by Dobzhansky and Meyer. Recently, 
Carl Woese has also proposed the animal 
classification
 Traditionally, depending upon presence or absence of the notochord, the animal 
kingdom has been divided into two groups- Non-chordates and Chordates.
History of animal classification
 Non-chordates are classified / divided into ten phyla. Those phyla are- Protozoa, 
Porifera, Coelentarata / Cnidaria, Platyhelminthes, Aschelminthes, Annelida, 
Arthropoda, Mollusca, Echinodermata and Hemichordata
Traditional method of animal classification
A. Non-Chordates : Characters of non-chordate animals are as follows
1. Body is not supported by rod-like notochord.
2. Pharyngeal gill-slits are absent.
3. Nerve cord; if present, it is on ventral side. It is solid & paired.
4. Heart, if present, it is on dorsal side.
63
 This system of animal classification was in practice till now. However, now a days, new 
system of classification is followed. We will study this new system of animal classification 
in brief.
 At present, according to the five kingdom classification system of Robert Whittaker, all 
multicellular animals are included in Kingdom: Animalia. This system of classification is 
based upon some criteria like Body organization, Body symmetry, Body cavity, Germinal 
layers, Segmentation, etc.
 All chordate animals are grouped together in a single phylum and the name of 
phylum is same i.e. Phylum- Chordata. This phylum has been divided into three 
subphyla as- Urochordata, Cephalochordata & V ertebrata. Sub-phylum V ertebrata has 
been further divided into six classes as- Class: Cyclostomata, Class: Pisces, Class: 
Amphibia, Class: Reptilia, Class: Aves and Class: Mammalia.
Do you know?
1.Cyclostomata
2. Pisces
3. Amphibia
4. Reptilia
5. Aves
6. Mammalia 
 
1. Protozoa
2. Porifera
3. Cnidaria
4.Platyhelminthes
5.Aschelminthes
6. Annelida
7. Arthropoda
8. Mollusca
9. Echinodermata 
10. Hemichordata  
1. Urochordata 
Chordates
Non-
chordates
Kingdom:
Animalia
3. Vertebrata
2. Cephalochordata
Phylum-
Chordata
Subphylum
Class  
6.3 Conventional System of  Animal Classification
Chordates : Characters of chordates are as follows
1. Body is supported by notochord.
2. Pharyngeal gill-slits or lungs are present for respiration.
3. Nerve cord is present on dorsal side of body. It is hollow. 
4. Heart is present on ventral side of body.
Phylum
 Notochord is a long rod like 
supporting structure present on 
dorsal side of animal body. It 
keeps the nerve tissue isolated from 
remaining body.
Sub Kingdom
Sub 
Kingdom:
Anus
Tail fin
Muscle
6.2 Characteristics of Chordates  
Notochord
Nerve chord
Mouth 
Gill slits 
64
Gonads
Intestine 
Mouth 
Bell  
Tube  
Muscle ring 
Tentacle  
Oral arms 
Jellyfish 
Criteria for new system of classification
A. Grades of organization        
 Body of animals is made up of cells. In 
case of multicellular animals, many cells 
are performing different functions in their 
body.  
 In case of multicellular 
animals, if tissues are not formed, 
their body organization is called 
as ‘Cellular grade organization’. 
Ex. Phylum-Porifera. 
 In case of some animals, 
cells come together to form 
tissues with the help of which all 
the body functions are performed. 
Such animals show ‘Cell - tissue 
grade’ organization. Ex. Animals 
from phylum- Cnidaria.
 Flat worms show ‘Tissue-
Organ grade’ organization. In 
this type of organization, tissues 
are organized to form some 
organs. However, complete 
organ systems are not formed.
Paramecium 
6.4 Protoplasmic-grade organization.
6.5 Cellular grade organization 
Gastrovascular
Cavity   
Planaria 
Amoeba
6.6 Cell -Tissue grade organization 6.7 Tissue -Organ grade organization 
Pharynx  
Nerve chord  Ganglia
Collar cells 
Amoebocyte 
Non living 
part 
Outlet (Osculla)
Ostium 
Sycon
Mouth
Spicule
Eye spot
In unicellular organisms all the 
functions of life are performed by same 
cell. Body organization of these unicellular 
organisms is called 'Protoplasmic grade' 
of organization
Do you know?
Page 5


61
Which criteria are used for classification of organisms?
6. Animal Classification
Ø History of animal classification
Ø New (method) system of animal classification.
Ø Kingdom- Animalia.
 You have studied the classification of living organisms in earlier classes. The living 
organisms present around us are mainly plants and animals. We have studied the criteria 
of their classification. With the help of that, complete the following chart.  
  Kingdom: Fungi 
 Kingdom: Plantae 
  Kingdom: Animalia
Kingdom: Monera
Multicellular
 organisms
Unicellular 
organisms
Kingdom: -----------
Living organisms
 How the plants are classified?
  We have studied the plants classification in last year. It helped us to understand the 
diversity of plants around us.
 You must be looking varieties of animals around you. Some animals are too small 
whereas some are too big. Some animals are terrestrial where as some are aquatic. Some 
animals crawl on land, some swim in water whereas some fly in air. Some animals have 
scales on skin whereas some have feathers or hairs. In this way, there is huge diversity 
among the animals too. According to recent studies, estimated number of animal species 
on earth is approximately 7 million. It is impossible to study each and every species. 
However, if groups and sub-groups of animals are formed depending upon the similarities 
& differences, it will make it very easy to study such vast variety of animals.
6.1  Classification of living organisms
Can you recall?
Can you recall?
62
 Formation of groups and sub-groups of animals depending upon similarities and 
differences among animals is called as animal classification. 
Benefits of animal classification
1. Study of animals becomes 
convenient.
2. Study of few animals from a 
group helps to understand about 
that entire animal group.
3. It gives idea about animal 
evolution.
4. Animals can be easily identified 
with great accuracy.
5. It helps to understand the 
relationship of animals with 
other living organisms.
6. It helps to understand the 
habitat of each animal and it's 
exact role in the nature.
7. It helps to understand various 
adaptations shown by animals.
 Time to time, different scientists have tried 
to classify the animals. Greek philosopher 
Aristotle was the first to perform the animal 
classification. Aristotle classified the animals 
according to the criteria like body size, 
habits and habitats. Further, as per the new 
developments in sciences, references were 
changed and thereby the criteria of animal 
classification too. Classification proposed 
by Aristotle is known as ‘Artificial method’. 
Besides Aristotle, artificial method of 
classification was followed by Theophrastus, 
Pliny, John Ray, Linnaeus, etc. Later on, 
‘Natural system of classification’ was followed. 
Natural system of classification was based on 
various criteria like body organization, types of 
cells, chromosomes, bio-chemical properties, 
etc. By the time, system of classification based 
on evolution was also brought into practice. It 
was used by Dobzhansky and Meyer. Recently, 
Carl Woese has also proposed the animal 
classification
 Traditionally, depending upon presence or absence of the notochord, the animal 
kingdom has been divided into two groups- Non-chordates and Chordates.
History of animal classification
 Non-chordates are classified / divided into ten phyla. Those phyla are- Protozoa, 
Porifera, Coelentarata / Cnidaria, Platyhelminthes, Aschelminthes, Annelida, 
Arthropoda, Mollusca, Echinodermata and Hemichordata
Traditional method of animal classification
A. Non-Chordates : Characters of non-chordate animals are as follows
1. Body is not supported by rod-like notochord.
2. Pharyngeal gill-slits are absent.
3. Nerve cord; if present, it is on ventral side. It is solid & paired.
4. Heart, if present, it is on dorsal side.
63
 This system of animal classification was in practice till now. However, now a days, new 
system of classification is followed. We will study this new system of animal classification 
in brief.
 At present, according to the five kingdom classification system of Robert Whittaker, all 
multicellular animals are included in Kingdom: Animalia. This system of classification is 
based upon some criteria like Body organization, Body symmetry, Body cavity, Germinal 
layers, Segmentation, etc.
 All chordate animals are grouped together in a single phylum and the name of 
phylum is same i.e. Phylum- Chordata. This phylum has been divided into three 
subphyla as- Urochordata, Cephalochordata & V ertebrata. Sub-phylum V ertebrata has 
been further divided into six classes as- Class: Cyclostomata, Class: Pisces, Class: 
Amphibia, Class: Reptilia, Class: Aves and Class: Mammalia.
Do you know?
1.Cyclostomata
2. Pisces
3. Amphibia
4. Reptilia
5. Aves
6. Mammalia 
 
1. Protozoa
2. Porifera
3. Cnidaria
4.Platyhelminthes
5.Aschelminthes
6. Annelida
7. Arthropoda
8. Mollusca
9. Echinodermata 
10. Hemichordata  
1. Urochordata 
Chordates
Non-
chordates
Kingdom:
Animalia
3. Vertebrata
2. Cephalochordata
Phylum-
Chordata
Subphylum
Class  
6.3 Conventional System of  Animal Classification
Chordates : Characters of chordates are as follows
1. Body is supported by notochord.
2. Pharyngeal gill-slits or lungs are present for respiration.
3. Nerve cord is present on dorsal side of body. It is hollow. 
4. Heart is present on ventral side of body.
Phylum
 Notochord is a long rod like 
supporting structure present on 
dorsal side of animal body. It 
keeps the nerve tissue isolated from 
remaining body.
Sub Kingdom
Sub 
Kingdom:
Anus
Tail fin
Muscle
6.2 Characteristics of Chordates  
Notochord
Nerve chord
Mouth 
Gill slits 
64
Gonads
Intestine 
Mouth 
Bell  
Tube  
Muscle ring 
Tentacle  
Oral arms 
Jellyfish 
Criteria for new system of classification
A. Grades of organization        
 Body of animals is made up of cells. In 
case of multicellular animals, many cells 
are performing different functions in their 
body.  
 In case of multicellular 
animals, if tissues are not formed, 
their body organization is called 
as ‘Cellular grade organization’. 
Ex. Phylum-Porifera. 
 In case of some animals, 
cells come together to form 
tissues with the help of which all 
the body functions are performed. 
Such animals show ‘Cell - tissue 
grade’ organization. Ex. Animals 
from phylum- Cnidaria.
 Flat worms show ‘Tissue-
Organ grade’ organization. In 
this type of organization, tissues 
are organized to form some 
organs. However, complete 
organ systems are not formed.
Paramecium 
6.4 Protoplasmic-grade organization.
6.5 Cellular grade organization 
Gastrovascular
Cavity   
Planaria 
Amoeba
6.6 Cell -Tissue grade organization 6.7 Tissue -Organ grade organization 
Pharynx  
Nerve chord  Ganglia
Collar cells 
Amoebocyte 
Non living 
part 
Outlet (Osculla)
Ostium 
Sycon
Mouth
Spicule
Eye spot
In unicellular organisms all the 
functions of life are performed by same 
cell. Body organization of these unicellular 
organisms is called 'Protoplasmic grade' 
of organization
Do you know?
65
Besides the four types of body organizations 
mentioned above, Remaining all animals show 
‘Organ - system grade organization’ in which 
different organs are joined together to form organ 
system that performs specific functions. Ex. Crab, 
Frog, Human, etc.
B.  Body Symmetry
 Take the pictures of human body and spongilla 
and try to take an imaginary section through 
specific plane of their bodies so as to get two equal 
halves.
 What did you observe? 
Observe
Body organization of human has been shown in the following figure. 
Use appropriate labels in that figure. Which organs are present in 
human body?
Radial Symmetry 
Asymmetrical body
6.9  Types of body symmetry
Mesoderm 
Endoderm 
Ectoderm 
Dead layer  
6.10 Diploblastic and triploblastic
C.  Germ Layers: Diploblastic and triploblastic
 In imaginary sense, if body of any animal is cut through imaginary axis of body, 
it may or may not produce two equal halves. Depending upon this property, there are 
different types of animal bodies.
Asymmetrical Body : In case of such body, there is no any such imaginary axis of the 
body through which we can get two equal halves. Ex. some sponges.
Radial symmetry : In this type of body, if imaginary cut passes through central axis but 
any plane of body, it gives two equal halves. Ex. Star fish. In case of this animal, there are 
five different planes passing through central axis of body through which we can get two 
equal halves.
Bilateral symmetry: In this type of body, there is only one such imaginary axis of body 
through which we can get two equal halves. Ex. Insects, fishes, frog, birds, human, etc. 
 In case of multicellular animals, germ layers 
are formed during initial period of their embryonic 
development and from those germ layers only, 
different tissues are formed in the body. In case of 
some animals, only two germ layers [Endoderm & 
ectoderm] are formed. Ex.: All Cnidarians. In most 
of all the remaining animals, three germ layers are 
formed i.e. mesoderm besides endoderm & ectoderm.
6.8 Organ- System  grade of organization 
Bilateral Symmetry