Characteristic features of different classes mentioned are as follows
(a) Chondrichthyes
(i) These are marine animals with a stream lined body.
(ii) Body is has cartilaginous exoskeleton and the skin is tough due to the presence of minute placoid scales.
(iii) These animals are predaceous.
(iv) Due to the absenceof air bladder they have to swim constantly to avoid sinking.
(v) They are cold blooded animals e.g., Scoliodon (dog fish), Pristis (sawfish).
(vi) Some of them possesselectric organs and poison sting (Torpedo and Trygon).
Osteichthyes
(i) These animals are found in both habitats, /.e., marine and freshwater. They bear bony endoskeleton.
(ii) Body is streamlined, posseses four pairs of gills covered by an operculum, on each side.
(iii) Skin is covered by cycloid or ctenoid scales.
(iv) Presence of air bladder is an additional feature providing buoyancy to animal.
(v) Heart is two chambered.
(vi) Sexes are seperate, fertilisation usually external, oviparous animal with direct development.
e.g., marine- Exocoetus (flying fish), Hippocampus (sea horse); freshwater Labeo {rohu).
(b) Urochordata
(i) In urochordates, notochord is only present in larval tail.
(ii) They contain a tough outer covering called tunic.
(iii) Persence of pharyngeal slits during some stage of the life cycle and a muscular, post ana ltail is the characteristic features in urochordates.
e.g., Salpa, Ascidia, Doliolum.
Cephalochordata
(i) Cephalochordates are exclusively marine animals.
(i) The notochord in cephalochordates extends, from the heart to tail region and is persistent throughout the life.
(iii) Body-wall shows one cell thick, non-ciliated epidermis, connective tissue, striated muscle and parietal peritonium.
(iv) Fertilisation is external
e.g., Branchiostoma (Amphioxus or Lancelet).
