1. Traditional interpretation

It suggests that the ancestral polychaetes gave rise to modern polychaetes through adaptive radiation. Then a group of annelids adapted in freshwater. The fresh water developed the ability to regulate the salt and water content of body fluids. Then the Oligochaetes are evolved from this group. Then some of the early Oligochaetes have to the Hirudinea.

2. Cladistic analysis

Cladistics analysis suggests that the phylum Annelida is not a monophyletic suggests that:

(a) Evolution of polychaetes: The Polychaetes arose from a metameric ancestor. It has independently of the Oligochaetes and leeches.

(b) Evolution of oligochoetes and leeches: The oligochoetes and leeches form a single They have many common important characteristics. A clitellum is present in groups. No unique synapomorphies (derived characteristics) are present in the oligochaetes. Rather, oligochaetes are defined by the absence of leech characteristics.  These facts support the idea that the oligochoetes and leeches should be combined into a single group called Clitellata. It means that the Polychaeta, Clitellata. Arthropoda and Pogonophora have a common ancestor. Their ancestor is a metameric species. Therefore, phylum annelida should be discarded.

3. Discovery of Archiannelida

A group of annelids hat e been discovered recently. It is called Archiannelida. These annelids lack coelom, setae, and some other annelid characteristics. The group is poorly known. Its taxonomic relationships to other annelids can not be established. Some of these worms are closed ancestral form. Some are derived from the polyphyletie lineage. Work is still going on these animals.



4. Conclusion

The traditional interpretation of later annelid evolution is still accurate. The evolutionary history of the polychaetes shows man) adaptive radiations. A few polychaetes adapted to freshwater environments. During Cretaceous period, (100 million years ago) oligochaetes came on moist, terrestrial environments. There was climax of the giant land reptiles in this period. It was the time of evolution of flowering plants. The modern oligochaetes consume deciduous  vegetation. It indicates that their ancestors also consumed the same food resource. A few oligochoetes secondarily invaded marine environments. Some of the early freshwater oligochaetes gave rise to the leeches. These leeches colonized marine and terrestrial habitats from freshwater.

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