organisms are structurally and functionally different but yet they possess
certain common fundamental features such as the arrangement of cell layers, the
levels of organisation, nature of coelom, the presence or absence of
segmentation, notochord and the organisation of the organ system.
members of Kingdom Animalia are metazoans (multicellular animals) and exhibit
different patterns of cellular organisation. The cells of the metazoans are not
capable of independent existence and exhibit division of labour. Among the
metazoans, cells may be functionally isolated or similar kinds of cells may be
grouped together to form tissues, organ and organ systems.
basic level of organisation is seen in sponges. The cells in the sponges are
arranged as loose aggregates and do not form tissues, i.e. they exhibit
cellular level of organisation. There is division of labour among the cells and
different types of cells are functionally isolated. In sponges, the outer layer
is formed of pinacocytes (plate-like cells that maintain the size and structure
of the sponge) and the inner layer is formed of choanocytes. These are
flagellated collar cells that create and maintain water flow through the sponge
thus facilitating respiratory and digestive functions.
animals, cells that perform similar functions are aggregated to form tissues.
The cells of a tissue integrate in a highly coordinated fashion to perform a
common function, due to the presence of nerve cells and sensory cells. This
tissue level of organisation is exhibited in diploblastic animals like
cnidarians. The formation of tissues is the first step towards evolution of
body plan in animals. (Hydra -
kinds of tissues aggregate to form an organ to perform a specific function.
Organ level of organisation is a further advancement over the tissue level of
and appears for the first time in the Phylum Platyhelminthes and seen in other
efficient and highest level of organisation among the animals is exhibited by
flatworms, nematodes,annelids, arthropods, molluscs, echinoderms and chordates.
The evolution of mesoderm in these animals has led to their structural
complexity. The tissues are organised to form organs and organ systems. Each
system is associated with a specific function and show organ system level of
organisation. Highly specialized nerve and sensory cells coordinate and integrate
the functions of the organ systems, which can be very primitive and simple or
complex depending on the individual animal. For example, the digestive system
of Platyhelminthes has only a single opening to the exterior which serves as
both mouth and anus, and hence called an incomplete digestive system. From
Aschelminthes to Chordates, all animals have a complete digestive system with
two openings, the mouth and the anus.
the circulatory system is of two types, the open type: in which the blood remains filled in tissue spaces due
to the absence of blood capillaries. (arthropods, molluscs, echinoderms, and
urochordates) and the closed type:
in which the blood is circulated through blood vessels of varying diameters
(arteries, veins, and capillaries) as in annelids, cephalochordates and
embryonic development, the tissues and organs of animals originate from two or
three embryonic germ layers. On the basis of the origin and development,
animals are classified into two categories: Diploblastic and Triploblastic.
in which the cells are arranged in two embryonic layers (Figure 2.1), the
external ectoderm, and internal endoderm are called diploblastic animals. In these animals the ectoderm gives rise to
the epidermis (the outer layer of the body wall) and endoderm gives rise to
gastrodermis (tissue lining the gut cavity). An undifferentiated layer present
between the ectoderm and endoderm is the mesoglea. (Corals, Jellyfish, Sea
Animals in which the developing embryo has three germinal layers are called triploblastic animals and consists of outer ectoderm (skin, hair, neuron, nail, teeth, etc), inner endoderm (gut, lung, liver) and middle mesoderm (muscle, bone, heart). Most of the triploblastic animals show organ system level of organisation (Flat worms to Chordates).
is the body arrangement in which parts that lie on opposite side of an axis are
identical. An animal’s body plan results from the animal’s pattern of
development. The simplest body plan is seen in sponges (Figure 2.2). They do
not display symmetry and are asymmetryical.
Such animals lack a definite body plan or are irregular shaped and any plane
passing through the centre of the body does not divide them into two equal
halves (Sponges). An asymmetrical body plan is also seen in adult gastropods
animals have paired body parts that are arranged on either side of a plane
passing through the central axis. When any plane passing through the central
axis of the body divides an organism into two identical parts, it is called radial symmetry. Such radially
symmetrical animals have a top and bottom side but no dorsal (back) and ventral
(abdomen) side, no right and left side. They have a body plan in which the body
parts are organised in a circle around an axis. It is the principal symmetry in
diploblastic animals. Cnidarians such as sea anemone and corals (Figure 2.3)
are radially symmetrical. However, triploblastic animals like echinoderms
(e.g., starfish) have five planes of symmetry and show Pentamerous radial symmetry.
which possess two pairs of symmetrical sides are said to be biradially symmetrical (Figure 2.4).
Biradial symmetry is a combination of radial and bilateral symmetry as seen in
ctenophores. There are only two planes of symmetry, one through the
longitudinal and sagittal axis and the other through the longitudinal and
transverse axis. (e.g., Comb jellyfish – Pleurobrachia)
which have two similar halves on either side of the central plane
an advantageous type of symmetry in triploblastic animals, which helps in
seeking food, locating mates and escaping from predators more efficiently.
Animals that have dorsal and ventral sides, anterior and posterior ends, right
and left sides are bilaterally symmetrical and exhibit cephalisation, in which
the sensory and brain structures are concentrated at the anterior end of the
animal (Figure 2.6).
presence of body cavity or coelom is important in classifying animals. Most
animals possess a body cavity between the body wall and the alimentary canal,
and is lined with mesoderm.
which do not possess a body cavity are called acoelomates. Since there is no
body cavity in these animals their body is solid without a perivisceral cavity,
this restricts the free movement of internal organs. (e.g., Flatworms)
animals, the body cavity is not fully lined by the mesodermal epithelium, but
the mesoderm is formed as scattered pouches between the ectoderm and endoderm.
Such a body cavity is called a pseudocoel
and is filled with pseudocoelomic fluid. Animals that possess a pseudocoel are
called pseudocoelomates e.g., Round worms. The pseudocoelomic fluid in the
pseudocoelom acts as a hydrostatic skeleton and allows free movement of the
visceral organs and for circulation of nutrients.
Eucoelom or true coelom is a fluid-filled
cavity that develops within the mesoderm
and is lined by mesodermal
Such animals with a true body cavity are called coelomates
or eucoelomates. Based on the mode of formation of coelom, the eucoelomates are
classified into two types, Schizocoelomates
– in these animals the body cavity is formed by splitting of mesoderm. (e.g.,
annelids, arthropods, molluscs). In Enterocoelomate
animals the body cavity is formed from the mesodermal pouches of
archenteron. (e.g., Echinoderms, hemichordates and chordates) (Figure 2.7).
animals, the body is externally and internally divided into a series of
repeated units called segments with a serial repetition of some organs
(Metamerism). The simplest form of segmentation is found in Annelids in which
each unit of the body is very similar to the next one. But in arthropods
(cockroach), the segments may look different and has different functions.
Animals which possess notochord at any stage of their development are called chordates. Notochord is a mesodermally derived rod like structure formed on the dorsal side during embryonic development in some animals. Based on the presence or absence of notochord, animals are classified as chordates (Cephalochordats, Urochordates, Pisces to Mammalia) and nonchordates (Porifera to Hemichordata).