a. Chondrichthyes and ostichthyes
• Their habitat is mainly marine and their body is streamlined. They have cartilaginous endoskeleton.
• Mouth is ventrally (near the front of the body) located.
• Notochord is persistent throughout the life.
• The air bladder is absent due to which they have to swim constantly to prevent sinking.
• They have tough skin, containing placoid scales. The teeth are modified placoid scales directed backwards.
• Heart is 2-chambered and they are cold blooded.
• Gill slits are separated and without operculum (cover for gills)
• Examples – Scoliodon, Pristis , Trygon
• The habitat is both marine as well as freshwater and they have a streamlined body. The endoskeleton is bony.
• Mouth is mostly terminal.
• Air bladder is present which gives buoyancy.
• There are 4 pairs of gills with operculum present.
• They are cold blooded.
• Heart has 2 chambers (1 auricle and 1 ventricle).
• Skin is covered with cycloid/ctenoid scales.
• Examples – Marinee : Hippocampus (Sea horse), Freshwater : Labeo (Rohu), Aquarium : Betta (Fighting fish)
b) Urochordata and cephalochordata
• Phylum chordata is divided into three categories – urochordata, cephalochordata and vertebrate.
• Urchordata and cephalochordate are often referred to as protochordates.
• They are exclusively marine.
• In urochordates, the notochord is present only in larval tail but in cephalochordates, notochord extends from head to tail region and is persistent throughout the life.
• Examples – Urochordates: Ascidia , Salpa , Cephalochordates : Branchiostoma.
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