The Sea Shore

by William S. Furneaux

Omitting all mention of sea birds, for the reason previously given, we now pass to the highest division of vertebrates—the Mammals—of which we shall describe but one species—the Common Porpoise, this being the only marine mammal that can be regarded as a frequent visitor to the British coasts in general.

It may be well at the outset to understand exactly why the porpoise is classed with the mammals and not with the fishes—to see how its structure and functions correspond with those of our own bodies rather than with those of the animals dealt with in the preceding portion of the present chapter.

First, then, while the young of fishes are almost invariably produced from eggs and are not nourished by the parents, the young of the porpoise are produced alive, and are nourished with milk secreted by the mammary glands of the mother. This is an all-important feature, and is the one implied in the term mammal. The porpoise also differs from nearly all fishes in that it breathes by lungs instead of gills, obtaining its air direct from the atmosphere, and not from the water. Hence we find it coming to the surface at frequent intervals to discharge its vitiated air and to inhale a fresh supply. The body-cavity of a mammal is divided into two parts by a muscular diaphragm, the foremost division, called the thorax, containing the heart and lungs, and the other (the abdomen) the remainder of the internal organs, while the diaphragm itself plays an important part in the respiratory movement by which air is drawn into the lungs. The body of the porpoise is so divided, but no such division ever occurs in any of the fishes. Lastly, the heart of the porpoise, in common with the rest of the mammals, is divided into four cavities, and the blood is warm, while the heart of a fish has generally only two divisions, and the blood propelled by it is of about the same temperature as that of the surrounding medium. Several other important differences between the porpoise and the fish might be given, but the above will be quite sufficient to show why they are placed in different classes.

Mammals are divided into several classes, and one of these (Cetacea) includes the fish-like Whales, Porpoises, and Dolphins, all of which are peculiarly adapted to a purely aquatic life. Like most of the fishes, their upper surfaces are of a dark colour, and the lower very light. Their fore limbs are constructed on the same plan as those of the higher mammals, the bones of the arm being attached to a large shoulder-blade, and the hand formed of four or five well-developed fingers which are enclosed in skin, so that they constitute a paddle or flipper well adapted for propulsion through water. There is no collar-bone, however, and the fingers have no nails or claws. There are no hind limbs visible externally, but a rudimentary pelvic girdle forms a part of the internal skeleton. A dorsal fin exists, but this is merely an extension of the skin of the back, and is not supported by either bones or rays. The skin itself has no scales, like that of most fishes, but is smooth and naked; and below it lies a large amount of fat, which, being a very bad conductor of heat, serves to prevent the escape of heat from the body.

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.