Placoid scales are tiny, hard, tooth-like structures, also called dermal denticles. They have the same structure as a tooth, having an outer layer of enamel, dentine and a central pulp cavity. Sharks' teeth and skin are composed of placoid scales. The whale shark and the megamouth shark are filter feeders that eat plankton. POD A pod is a social group of whales.
Baleen whales travel alone or in small pods.
Reptiles are poikilothermic. A polyphyletic group consists of organisms but not their common ancestors. This is an artificial group which is based primarily on physical characteristics rather than on evolutionary relationships. It must swim continuously to get oxygen. They live in the North and South Atlantic and the South Pacific Oceans.