The tails of cetaceans are also mere folds of the skin, supported in the centre by the extremity of the vertebral column; but unlike the tail fins of fishes, they are expanded horizontally instead of in the vertical plane. This latter is an important adaptive feature of the cetaceans, since the vertical movement of a tail so disposed is exactly what is required to assist the animals as they alternately rise to the surface for air and again descend into the sea in search of their food. Among the other external characters of the cetacean we may note the nostrils, which are always situated on the highest point of the head, and are thus the first part exposed when the creature rises to renew its supply of air; also the ears, which are two small apertures behind the eye, without any form of external appendages. The skeleton of the cetacean is formed of light spongy bones, saturated with oily matter; and although the animal has no true neck, visible as such externally, it is interesting to note that, in common with all other mammals, even with the long-necked giraffe, it possesses its seven cervical or neck vertebræ. Porpoises and Dolphins together form the family _Delphinidæ_, characterised by having the blow-hole in the form of a crescent with its convexity turned towards the front, and of these the Porpoises constitute the genus _Phocæna_. The Common Porpoise (_P. communis_) is the species that is so often seen close to our shores and in the harbours and estuaries, swimming in shoals with a graceful undulatory movement. Porpoises move forward entirely by the vertical action of their powerful horizontal tails, and extend their flippers only to change their course or to arrest their progress. At short intervals they rise to the surface, exposing their slate-coloured backs and dorsal fins for a moment, and then immediately dive downwards in such a manner as to appear to turn a series of somersaults. Occasionally they will leap quite out of the water, exhibiting their white under surfaces, which shine with a sudden flash when illuminated by the rays of a bright sun. The blow-hole is the first part exposed, and if one is sufficiently near the shoal a fountain of spray may be seen to shoot into the air, and the outrush of the expired air may be heard as each one makes its appearance. [Illustration: FIG. 244.--THE COMMON PORPOISE]