I: | Four pairs of gills covered by an operculum on each side |
II: | Skin covered with placoid scales |
III: | Regulation of buoyancy by an air bladder |
I: | They are exclusively marine. |
II: | They have cellular level of organisation. |
III: | They have a water vascular system with tube feet. |
IV: | Digestion is intracellular. |
V: | Fertilisation is external and development is direct. |