We are now beginning to deal with features in which there are considerable and obvious divergences between the groups and these can best be described by noting them in the pinnipedes and then briefly contrasting them with what appears in the other two orders. Unlike the Cetacea and Sirenia which are entirely aquatic throughout life, the Pinnipedia have not lost all contact with the land. Some spend more time in the water than others, but all come to land (or ice) for breeding and for basking between feeding bouts and therefore have retained an ability to move on land. This is achieved by two different methods, one found in the true seals and the other in the fur-seals, sea-lions and walruses (Fig. 1). In these latter the hind-limbs are still capable of being directed forwards and acting as a foot, albeit on an extremely short leg which is buried in blubber down to the ankle. Thus with the associated flexure of the trunk in the lumbar region they can move rapidly over a land surface. The true seals, however, have hind-limbs which are directed backwards and can only trail on land. Movement is therefore much more laboured in a terrestrial habitat. A comparison between the limbs of a furseal or sea-lion and those of a true seal shows clearly the very considerable differences not only in structure and posture, but also in use.
The fore-limbs of a fur-seal are long and on land are capable of reflexion between the wrist and palmar surface. The forearm and wrist form a vertical prop as it were, while the hand, supported by the meta-carpal and digital bones, lies flat on the ground (Fig. 2a). The web between the digits is thin, in fact the whole of the distal part is much thinner and longer than in the true seals. This is associated with the much greater use which the fur-seals make of their fore-limbs in swimming. The claws are quite rudimentary and in some almost missing altogether. In the true seals the fore-arm is buried in blubber, and only the wrist and palmar surface with short digits protrude as short flaps (Fig. 1). The webbing cannot be distinguished as such for the digits are united by thick tissue so that the separate digital lines are not visible externally. The claws are strong and used for grooming the surface of the body. In the water these limbs are used principally for changing direction or for slow paddling, never for rapid movement, when they are held tightly pressed against the flanks. The digits still retain, as Backhouse has shown, an ability to flex with considerable power so that the animal can haul itself up over boulders and rocks and to some extent compensates for the loss of power of the hind-limbs. At any rate this is true for the northern true seals. In the antarctic species this ability if present is not used, so far as observations made by a number of observers appear to confirm (O’Gorman).
The hind-limbs of a fur-seal are also long but can be directed forwards for movement on land. They are also used on land during basking as fans. When fur-seals are hauled-out for breeding the bulls have to remain on their territories for a long time and they soon dry out. Their layer of blubber and the thick undercoat of fur, which keeps them warm in water, is now a disadvantage when the sun comes out. They then use the hind flippers as huge fans, turning the body on one side so that both flippers can fan the anterior part of the body and head which has less fur on it (Pl. 1). The claws here are usually absent altogether or quite rudimentary on the first and fifth digit but well developed in the middle three which can thus be used for grooming. In the true seals the hind-limbs are permanently directed backwards and form powerful sculling organs (Fig. 2). The palmar surfaces of the feet are turned inwards to face each other. The digits are united by thin and extensive webbing and the digits themselves are strong, the outer ones (the first and fifth) being much longer and thicker than the others. By alternate sweeps from side to side in a sculling movement these flippers are able to drive the seal through the water at great speed which has been estimated at as much as 12 to 15 m.p.h. Claws are retained on all the digits but their function is obscure. On land these hind flippers trail behind and take no part in locomotion which is achieved by a ‘humping’ motion. The body is flexed and the pelvic region advanced to give a forward push to the anterior part of the body, the stomach and the chest taking the main load as the animal moves forward. The fore-limbs are not used in this motion unless obstacles have to be surmounted when they assist in dragging the body upward, helped by the thrust of the pelvis. Thus in the Pinnipedia there are two distinct means of progression both on land and in water.
In the Cetacea the principal locomotor organ is the tail, flattened dorso-ventrally into a triangular fluke, and moved up and down in a sort of ‘crawl’ action. The hind-limbs are missing and the fore-limbs are well developed into flippers. Not only are the digits joined by thick webbing but the number of phalangeal bones is greatly increased so that the flipper becomes extremely flexible in its use as a propulsive organ and also in causing change in direction. The Sirenia are rather lethargic and movement is obtained by the action of the flattened tail and by the broad, short fore flippers. The latter are peculiar in having a great power of rotation so that their direction of action can vary over a wide angle. This is of considerable advantage when used as a paddle, the animal being able to turn almost in its own length, or when erect in the water, in little more than its own breadth.
A feature associated with streamlining is the reduction of the pinna or external ear flap. In the Pinnipedia all variations are found; among the fur-seals and sea-lions it is still fairly well developed but narrow (furled) and elongate, lying back alongside the head. For this reason they are often called the ‘eared’ seals. In the true seals it is much reduced and does not extend beyond the dried hair. When the hair is wet it can just be seen, if the light is right, as a circular rim round the earhole (Pl. 1). In the walrus it is equally inconspicuous. In both Cetaceans and Sirenia it is completely absent and in the former the earhole is plugged. (Only recently has the extraordinary hearing mechanism of whales and porpoises been demonstrated by Drs Fraser and Purves.)
The normal resting closed position of the nostrils has already been mentioned as common to all three groups. It remains to point out that the pinnipedes retain a ‘rhinarium’ external to the nostrils. This is the equivalent of the ‘wet nose’ of the dog and must constitute an important sense organ for the reception of chemical stimuli (scent). That it has to operate in water is no drawback because the molecules of the ‘scent’