CLASSIFICATION OF HEMICHORDATA & CHORDATA
Hemichordates are widely distributed in shallow, marine and tropical waters. They are present in deep and cold waters. They have soft and wormlike bodies. They have epidermal nervous system. Most hemichordates have pharyngeal slits. They have three classes.
- They live in shallow water.
- They are worm like animals.
- They live in burrows on sandy shorelines.
- Their body is divided into three regions: proboscis, collar and trunk.
- They have about 70 species.
Example: Acorn worms (Balanoglossus, Saccoglossus)
- They are with or without pharyngeal slits.
- They have two or more arms.
- They are often colonial.
- They live in an externally secreted encasement.
- They have about 20 species
- They have spherical body. The body surface is covered by ciliary bands.
- They have U-shaped digestive tract.
- Coelom is poorly developed.
- They are planktonic.
- Only one species exist
Example: Planctosphaertara pelagica
- Notochord is present in them.
- They have pharyngeal slits.
- Dorsal tubular nerve cord is present in them.
- Postanal tail is developed at some time in the chordates life.
Thave bout 45,000 species. Phylum chordata have three sub phylums.
A. Subphylum Urochordata
Notochord, nerve and postanal tail is present only in free swimming larvae. The adults are sessile or planktonic. Their body is enclosed in a tunic that contains some cellulose. Mostly they are marine. Example: Sea squirts or tunicates.
1. Class Ascidiacea: They all are sessile as adults. They may be solitary or colonial. The colony members are interconnected by stolons.
2. Class Appendicularia (Larvacea): They are planktonic. The adults retain tail and notochord. They lack a cellulose tunic. The epithelium secretes a gelatinous covering of the body.
3. Class Sorberacea: They are ascidian-like urochordates. They possess dorsal nerve cords. They live in deep water. They are carnivorous. Example: Octacnemus
4. Class Thaliacae: They are planktonic. They adults are tailless and barrel shaped. The oral and atrial endings are present at opposite ends of the tunicate. The muscular – tractions of the body wall produce water currents.
B. Subphylum Cephalochordata
Their body is laterally compressed. Body is transparent and fishlike. All four chordate characteristics persist in them throughout life. Example: Amphioxus. It has about 45 species.
C. Subphylum Vertebrata
Notochord, nerve cord, postanal tail and pharyngeal slits are present at least in embryonic stages. The vertebrae surround the nerve cord. Vertebral column acts a primary axial support. Theirskeleton is modified anteriorly into a skull. Skull protects the brain. It has eight classes.
1. Class Cephalaspidomorphi: Their body is fishlike. They are jawless. They have no paired appendages. They have cartilaginous skeleton. They have sticking mouth with teeth and rasping tongue. Example: Lampreys.
2. Class Myxini: Their body is fish like. They are jawless. They have no paired appendages. Their mouth has four pairs of tentacles. Their olfactory sacs open to mouth cavity. They have 5 to 15 pairs of pharyngeal slits. Example: Hagfishes
3. Class Chondrichthyes: They are fishlike. The jaws are present. They have paired appendages and cartilaginous skeleton. They have no swim bladder. Examples: Skates, rays, sharks.
4. Class Osteichthyes: They have bony skeleton. They swim bladder and operculum is present. Example: Bony fishes
5. Class Amphibia: Their skin produces mucous. They possess lungs or gills. They have moist skin. Skin acts as respiratory organ. There are aquatic developmental stages in them. Larva is changed by metamorphosis into adult. Example: Frogs, toads, salamanders.
6. Class Reptilia: They have dry skin with epidermal scales. They have amniotic eggs. The y have terrestrial embryonic development. Example: Snakes, lizards, alligators.
7. Class Aves: They possess feathers. They efficiently regulate body temperature (endothermic). They have amniotic eggs. Example: All birds.
8. Class Mammalia: Their bodies are partially covered by hair. They are endothermic. The young are nursed by mammary glands. They have amniotic eggs. Mammals