FURTHER PHYLOGENETIC CONSIDERATIONS Relationship between Hemichordates and Chordates It is difficult to establish evolutionary relationships between the hemichordates and chordates. Two common characteristics are present between these groups: 1. Dorsal tubular nerve cord. 2. Pharyngeal slits of hemichordates. These two characteristics are the evidence
FURTHER PHYLOGENETIC CONSIDERATIONS
FURTHER PHYLOGENETIC CONSIDERATIONS The arthropods are very successful animals. They have diverse body forms. The lifestyles copepods, crabs, lobsters, crayfish, and barnacles demonstrate adaptive radiation. Few aquatic environments are without some crustaceans. Phylogeny of Chelicerates: The subphylum Chelicerata is a very important group of
FURTHER PHYLOGENETIC CONSIDERATIONS 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.
FURTHER PHYLOGENETIC CONSIDERATIONS Molluscs are over 500 million years old according to fossil record. There are different views about the evolution of molluscs. Protosotme ancestry The molluscs have protosotme ancestry. But zoologists do not know the exact relationship of this phylum with other animal
FURTHER PHYLOGENETIC CONSIDERATIONS The aschelminths are diverse animals. They have common cuticle, pseudocoelom, muscular pharynx and adhesive glands. But no distinctive features in every phylum. 1. Origin of Rotifers The rotifers and acoelomates have certain common features. The protonephridia of rotifers closely resemble with
FURTHER PHYLOGENETIC CONSIDERATIONS There are different views about the evolution of platyhelminthes: Zoologists believe that the platyhelminthes body form is central to animal evolution. They believe that an ancestral flatworm was similar to turbellarians. The cladogram of platyhelminthes unites the Monogenea, Trematoda and Cestoidea.
FURTHER PHYLOGENETIC CONSIDERATIONS Debate is still going on the position of these phyla. There are different views aboutthe origin of these phyla 1. Polyphyletic: It means that all phyla have separate origin. But still scientist that the number of independent origins is probably small.
FURTHER PHYLOGENETIC CONSIDERATIONS Fossilized protozoans: Protozoa originated about 1.5 billion years age. But there are only 30,000 known fossils of protozoans. These fossils are useless for study of evolution of the various protozoan groups. Only protozoa with hard parts (tests) are present among the