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The three germ layers ectoderm, mesoderm and endoderm are found in
Option: 1
Chordates only
Option: 2
Chordates except protochordates
Option: 3
All the chordates and higher invertebrates
Option: 4
Higher chordates and higher invertebrates
Answers (1)
Phylum Chordata -
The phylum chordata is characterized by the following:
Notochord:
- The name ‘Chordata’ is based on the presence of notochord at some stage or throughout the life cycle.
- It is a rod-like structure placed dorsally between the alimentary canal and nerve cord.
- It is derived from the mesoderm.
- In vertebrates, it is present during the embryonic development.
- It is then replaced by the vertebral column (spine) in the postnatal stage.
Dorsal Hollow Nerve Cord:
- It is placed dorsally to the notochord.
- It is hollow rod and is derived from the ectoderm.
- It develops into the brain and spinal cord and forms the central nervous system.
Pharyngeal Gill Slits:
- These are the openings present in the pharynx.
- They allow the entry and exit of water in aquatic animals.
- In vertebrate fishes, these develop into gills. In tetrapods, these are modified into tonsils and components of ears.
Post-anal Tail:
- It is the posterior elongation of the body that goes beyond the anus.
- It consists of skeletal elements and muscles.
- It helps in locomotion in fishes and in balancing, courting etc. in terrestrial vertebrates.
- In is reduced and non-functional in man.
-
Higher invertebrates like Platyhelminthes, aschelminthes, annelids, arthropods, molluscs, echinoderms and all chordates are triploblastic.
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