There are following views about animal taxonomy:

1. Monophyletic: Animalia have impressive similarities in animal cellular organization. “lherefore. they are considered as monophyletic. It is a traditional approach. It is followed by most of the scientists. However, there is an exception for sponges (phylum Porifera).

2. Polyphyletic: An evolutionary explosion occurred about 0.6 billion years ago at the beginning of the Cambrian period. It resulted in the origin of all modern phyla (along NA Id’ other animals that are now extinct). This rapid origin and diversification of animals is called the Cambrian explosion. It occurred over a relatively brief one hundred million-year period. This rapid divergence of many kinds of animals from a single ancestor seems unlikely. Therefore, some scientists started believing that animals may be polyphyletic. This idea is followed by a few scientists.

Taxonomic outlines of animals

Many zoologists recognize three major groups within the animal kingdom. These groups are called branches. These groups are:

1. Mesozoa: It includes phylum Mesozoa, the mesozoans.

2. Parazoa: It includes phylum Porifera (sponges).


3. Eumetazoa: It includes all other phyla. The Eumetazoa arc further divided into different groups. This grouping is based on body symmetry and embryological characteristics.

(a)        Classification based on symmetry: Eumetazoans are divided into two groups on the basis of symmetry. These are Radiata and Bilateria.

(b)   Classification based on embryology: In this system. the bilaterally symmetrical animals are divided into two large groups. This group is based on similarities and differences in the early development of the animals. The embryological cv ems may he similar because of shared ancestry. However. the embryologists must sort homologous developmental sequences from analogous developmental sequences. Animals are divided in two groups on the basis of embryological developmental statics.

(i)           Protostomes

The animals in which blastopore forms mouth and anus is formed as separate opening are called protostomes. It includes phyla Platyhelminths, Nematoda, Mollusca, Annelida, Arthropoda, and others. Protostomes shows spiral cleavage. In this case. the mitotic spindle is oriented obliquely to the axis of the zygote. This orientation produces an eight-celled embryo. In this embryo the upper tier of cells is twisted out of line with the lower cells. The cleavage of most of the protostomes is determinate. In this ease, the fate of the cells is established very early in development. If cells of the two- or four-celled embryo are separated, none develops into a complete organism. Protostomes have characteristics coelom and gut formation. Many protostomes develop top-shaped larva cal led trochophore larva.

(ii)         Deuterostomes

The animals in which blastopore forms anus and mouth arises as a separate opening are called deuterostomes. It includes phyla Echinodermata, Hemichordata, Chordata and others. The cleavage occurs in deuterostomes is radial. In this case, mitotic spindle is oriented perpendicular to the axis of the zygote. Thus embryonic cells directly conic come over one another. Their cleavage is indeterminate. In this case. the fate of blastomeres is determined late in development. If embryonic cells are separated. they can develop into entire individuals. The manner of gut tract and coelom formation differs from protostomes. Some deuterostomes possess a kidney-bean-shaped larva called a dipleurula larva. However, there is no single kind of deuterostomes larval stage.

Exception: A number of phyla (e.g.. Mollusca. Nematoda. and others) do not lit %Nell into deuterostome or protostome groups. Cladistic analyses and molecular approaches give alternative groupings. These groupings are becoming popular.






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