The cetaceans include the mysticetes
or baleen whales and the odonotocetes, which are the dolphins and
porpoises. The most obvious difference between mysticetes and odonotocetes
are in their feeding mechanisms. The mysticetes have a system for
filtering small organisms from the water, utilizing baleen suspended
from their upper jaw. The baleen consists of plates of keratin (the
same material that forms the hair and fingernails in other mammals),
which have a brush-like inside surface. The size and number of these
baleen plates and the thickness of each strand depends on the feeding
habits of that particular species. Another important characteristic
of the mysticetes is that they have two external blowholes for respiration.
Odontocetes do not have baleen; they have teeth in both jaws. The
size and shape of these teeth also depends on the feeding habits
of each species. In contrast to the mysticetes the odontocetes possess
only one external blowhole for breathing. The odontocetes also have
an oily mass that occupies the front of the cranium. This mass is
known as the melon and its function is to transmit and receive different
frequencies of sound for communication. In this way, the cranium
in the odontocetes acts something like sonar.
The mysticetes are very large. The blue whale, for example, is the
largest animal in the world, reaching 30 meters total length. The
odontocetes are more variable in size with the smallest, the Vaquita,
scarcely 1.5 meters long, and the largest sperm whale, with a length
of more than 18 meters.
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