This group includes animals with bilateria symmetry. In an animal where the right side is approximately the same as the left side and where there is a distinct anterior end is said to have bilateral symmetry. The animal can be divided into two equal parts by an imaginary line only in one plane. In most multcellular animals there is a clearly differentiated head present at the anterior end and a distinct posterior end.also there are clearly difined dorsal and ventral surfaces.

The animals belonging to phyla, platyhelminthes, Nematoda, Annelida, Mollusca, Arthropoda, Echinodermata, hemichordate and Chordata are included in this grade. It must be mentioned here that the animals belonging to phylum echinodermata, have developed bilateral symmetery, as is evident, from the study of structure of their larvae. However, the adult Echinoderms, have secondarily developed radial symmetry, due to their special mode of life. All the animals included in grade bilateria are triploblastic. These may be acoelomate, pseudocoelomate or coelomate.


Fig. 10.1 (a) Radial (b) bilateral symmetry

Series proterostomia (Protostomes) Series Deuterostomia: (deuterostomes)
  1. Cleavage or division of the zygote is spiral and determinate.
  2. During development process the mouth in these animals arises from the blastopore or from its anterior margin
  3. Coelom or body cavity is formed due to splitting of mesoderm (schizocoelous).
  4. Mesoderm is derived from cells on anterior lip of blastopore.
  5. This series proterostomia includes animals belonging to phyla aschelminthes (nematode) annelid, mollusca and arthropoda
  1. Cleavage is radial and indeterminate.
  2. During embryonic development mouth is fromed at some distance anterior to the blastopore and blastopore forms the anus.
  3. Coelom is developed as an outpouching (enterocoelous).
  4. Mesoderm is derived from wall of developing gut (archenteron).
  5. This series includes animals belonging to phyla echinodermata,hemichordate and chordate.



Fig. 10.2 Patterns of embryonic development of coelom and of egg cleavage in protostomes and deuterostomes.

A spiral and determinate cleavage is that in which the lines or planes of cleavage are not symmetrical between poles instead these are diagonal to the polar axis and produce unequal cells around the axis of polarity and all the balstomeres have determined role to play in the formations of embryo. The fate of each blastomere is foretold.

In radial and indeterminate cleavage the planes of cleavage are symmetrical to the polar axis and produce tiers of cells on top of each other and the fate of each blastomere is not pre-determined. In some anyone blastomere can produce a complete embryo.

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