Whales, derived from Proto-Germanic word hwæl,
are a widely distributed and diverse group of fully aquatic marine mammals. They comprise the extant families Cetotheriidae (whose only living member is the pygmy right whale), Balaenopteridae (the rorquals), Balaenidae (right whales), Eschrichtiidae (the gray whale), Monodontidae (belugas and narwhals), Physeteridae (the sperm whale), Kogiidae (the dwarf and pygmy sperm whale), and Ziphiidae (the beaked whales). There are 40 extant species of whales. The two suborders of whales, Mysticeti and Odontoceti, are thought to have split up around 34 million years ago. Whales belong to the clade Cetartiodactyla and their closest living relative is the hippo having diverged about 40 million years ago.
More words before the unorderd list
Several species exhibit sexual dimorphism, in that the females are larger than males. They have streamlined bodies and two limbs that are modified into flippers. Though not as flexible or agile as seals, whales can go at incredibly fast speeds, up to 20 knots. Balaenopterids (rorquals) use their throat pleats to expand their mouth to take in huge gulps of water. Balaenids have huge heads that can make up 40% of their body mass to take in huge amounts of water. Odontocetes have conical teeth designed for catching fish or squid. Mysticetes have a well developed sense of "smell", whereas Odontocetes have well-developed senses—their hearing that is adapted for both air and water, and can survive even if they're blind. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water.
But a few, like the sperm whale, feed on large invertebrates, such as giant squid. Grey whales are specialized for feeding on bottom-dwelling mollusks. Male whales typically mate with multiple female every year, but females only mate every two to three years. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively long period of time. Whales produce a number of vocalizations, notably the songs of the humpback whale.
This is a link to the course as described on Quora. Links are fun.