MULTICELLULAR & TISSUE LEVEL ORGANIZATION
The scientists and common people are much interested in the multicellular level of organization. The swimmers of the tropical water have found Physalia physalis. It is commonly is know as Portuguese man war. Such organisms are composed of group of cells. These groups of cells are specialized tbr various functions. These cells are independent. The simple multicellular organization includes three phyla: Phylum Porifera., Phylum Cnidaria and Phylum Ctenophora.
ORIGINS OF MULTICELLULARITY
Multicellular life has formed approximately 550 million years ago. But at that time there we are only 10% multicellular organisms. Multicellular life arise in Precambrian and Cambrian boundary. The scientists take this period as evolutionary explosion. All the animal phyla appeared during this evolutionary era. Now 15 to 20 groups of multicellular animals have become extinct. The multicellular animals have becoming extinct since their origin.
There is a mystery in the evolution of multicellularity. There are two hypotheses about the origin of multicellular organisms:
1. Colonial hypothesis: It describes that the dividing cells remained together. They formed multicellular colonies. These colonies are present in protista. It is called colonial hypothesis.
2. Syncytial hypothesis: A syncytium is a large multinucleate protist cell. Plasma membranes are formed in the cytoplasm of a syncytial protist. These membranes produced a small, multicellular organism.
Both the colonial and syncytial hypotheses are supported by the colonial and syncytial organization. These organizations are present in some protist phyla.
There are three views about the animal origins:
1. Monophyletic: It means that the multicellular animals had derived from a single ancestor. All animal phyla in fossils appeared simultaneously in the Precambrian and Cambrian boundary. Therefore, it is difficult to explain that animals arc monophyletic. But impressive similarities are present in animal cellular organization. It supports the view that all or most animals are derived from a single ancestor. For example:
(i) Asters formation during mitosis in most animals.
(ii) Certain cell junctions are similar in all animal cells.
(iii)Most animals produce flagellated sperm.
(iv)The proteins involved in movement are similar in most animal cells.
But it is difficult to explain the origin of these common features.Therefore, the scientists do not accept this view.
2. Diphyletic: It means animals derived from mo ancestors. This view is not much considered.
3. Polyphyletic: It means animals are derived from many ancestors. Most of the zoologists accept polyphyletic view. But more than one body forms can be arised from their ancestors. Therefore, one or two hypotheses about the origin of multicellularity can be correct.