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10. Kingdom Animalia eLearn.Punjab
DIPLOBLASTIC AND TRIPLOBLASTIC ORGANISATION
Diploblastic animals belong to division radiata. The body of these animals consists of two layers
of cells, ectoderm and endoderm. There is a jelly like mesenchyme or mesogloea which in most
cases is non cellular. Diploblastic animals show lesser degree of specialisation and they do not
form specialised organs. There is no special transport system in these animals. Most substances
are distributed within their body by process of diffusion. There is no central nervous system in
these animals. A neuron net is present. These animals have radial symmetry. There is only one
cavity in the body called gastrovascular cavity which has only mouth which serves for the entry
of food and water and also for the removal of wastes along with water. This is known as sac like
digestive system. Diploblastic animals are included in phylum Cnidaria (coelenterate) which would
be discussed in detail later in the chapter.
Triploblastic animals are included in phyla which have been placed in grade bilateria/ The body of
these animals is made from three layers ectoderm, mesoderm and endoderm. After embryonic
development these layers in most triploblastic animals are not distinct as separate layers of cells,
but are represented by the structures formed from them. The cells of these animals show greater
degree of specialisation. These have specialised organs and organs systems. Special transport
systems i.e. blood vascular system is present in most of the cases. The systems such as integumentary
and nervous develop from ectoderm. Mesodem gives rise to muscular, skeletal and reproductive
systems. Endoderm forms the lining of digestive tract and glands of digestive system, such as liver.
The digestive system is of tube type i.e. having mouth at the anterior end and the anus at the
posterior end. Triploblastic animals may be acoelomate, pseudocoelomate or coelomate.
Acoelomates, Pseudocoelomates and Coelomates
The following account would help to explain the above mentioned terms.
Acoelomates
In phylum Platyhelminthes there is no body cavity or coelom, and the mesoderm forms a loose,
cellular tissue called mesenchyma or parenchyma which fills the space between the ectoderm and
endoderm. It forms a packing around the internal organs of the animals to support and protect
them. Such animals are called acoelomates (Fig. 10.3). In acoelomates the gut is sac-type and there
is no special transport system. Only excretory system is developed for the transport of excretory
products. This system consists of flame cells, excretory ducts and excretory pores. However the
nervous system is well developed.
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