SEGMENTATION
In some animals, the body is externally and internally divided
into segments or somites with a serial repetition of atleast
some organs. Segmentation can be of following two types
1. Metameric Segmentation
A segmentation that simultaneously divides body both
externally and internally is called metamerism or
metameric segmentation. This kind of segmentation is
found in annelids, arthropods and chordates.
2. Pseudometamerism
It is found in tapeworm, the body is divisible into parts or
segments called proglottides. They develop from the neck
but are not embryonic in origin. Such a repetition which
appears due to repeated budding is known as false
segmentation or pseudometamerism.

Germ layers are group of cells behaving as a unit during
early stages of embryonic development. These germ
layers differentiate to give rise to all the tissues/organsof
the fully formed body of individuals.
On the basis of germ layers animals are classified as
follows
1. Diploblastic
Animals in which the cells are arranged in two
embryonic layers—an external ectoderm and an
internal endcxierm, are called diploblastic animals. In
addition, an undifferentiated layer, mesoglea is present
in between the ectoderm and the endoderm, e.g.,
coelenterates.
2. Triploblastic
The animals in which the developing embryo has a third
germinal layer mesoderm, in between the ectoderm and
endoderm are called triploblastic animals, e.g., all animals
from phylum—Platyhelminthes to phylum— Chordata.

LEVELS OF ORGANISATION
All members of kingdom— Animalia are multicellular. heterotrophic eukaryotes
But, all of them do not exhibit the same pattern of organisation of cells. The
cells in their body arc of types. These are organised into many functional
units of progressively increasing complexity.
The animal body shows four basic plan of structural organisation as given below
l. Cellular
In this level, the body shows some division of labour among cells. They are
remarkably independent and can change their form and function. It is found in
sponges. The body consists of many cells arranged as loose cell but,
the cells do not form tissues rather each cell functionally active independently,
2. Tissue Level
The cells performing the same function are arranged into
tissues, hence is called tissue level of organisation, e.g., in
coelenterates. the arrangement of cells is more complex.
3. Organ Level
In Platyhelminthes and other higher phyla, tissues are grouped
together to form organs. each specialised for a particular
function. i.e.. organ level organisation is present.
4. 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 is called organ system level of organisation.
Organ systems in different groups of animals exhibit various
patterns of complexities.
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 thus, called incomplete.
A complete digestive system has two openings, i.e., mouth and
anus.
Another example is the two types of circulatory systems. Open
type circulatory system, in which blood is pumped out of the
heart and cells and tissues are directly bath in blood.
In closed type system, the blood is circulated through a network
of blood vessels of varying diameters (arteries, veins and
capillaries) and tissues remain in touch with.

Disadvantages of artificial system are given below
(i) This system uses only few superficial characters (i.e.v
habits, numbers, colours and shapes of leaves, etc) which
leads to many organisms grouped together.
(ii) ‘Ihey considered mainly the vegetative characters or the
androecium characters as given by Linnaeus.

Bryophytes show alternation of generation. The main
plant body is gametophyte. After fertilisation of egg and
spermatozoids, zygotes do not undergo reduction division
and they produce a multicellular body called sporophyte.
Then some cells undergo division and produce haploid
spores which germinate to produce gametophyte.