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 specialization and they do
not form specialized organs. There is no special transport system is
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 specialization. These have specialized
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. Mesoderm 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 systm is of tube type i.e. having mouth at the anterior end
and the anus at the posterior end. Triploblastic animasl 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. If 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.
Pseudocoelomates
In
aschelminthes the space between the body wall and the digestive tube is
called pseudocoelom (false body cavity). Pseudocoelom is not homologous
to true coelom because: it is not lined by coelomic epithelium. It has
no relation with the reproductive and excretory organs. It develops from
the blastocoels of the embryo and it is bounded externally by the
muscles and internally by the cuticle of the intestine. The animals
having pseudocoelom are called pseudocoelomates.
Coelomates
Coelom
is cavity present between the body wall and the alimentary canal and is
lined by mesoderm. The mesoderm splits into outer parietal layer which
under lined the body wall and the visceral layer which covers the
alimentary canal and thecavity between them is the true coelom. It is
filled with with fluid called coelomic fluid. The animals which possess
coelom or true body cavity are called coelomates e.g. animals from
annelids to chordates.
In coelomates
gut attains more complexity and neuro-sensory system is well developed
along with excretory system, circulatory system, respiratory and
reproductive systems.
Fig. 10.3: General body plan of acoelomate, pseudocoelomate and coelomate.
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