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’ must be dissolved before they can stimulate a receptor nerve ending, for which reason the rhinarium of land mammals is always kept moist with secreted mucus. In the pinnipedes it is possibly the sense organ which locates prey, such as shoals of fish at a distance.
The teeth of pinnipedes are greatly modified from the normal mammalian pattern. The differentiation into incisors, canines and molars is still recognisable, but they do not differ so much from each other as in other (land) mammals, consisting basically of single pegs of varying lengths. The incisors are small and degenerate as shown by the variation in their number. (It is not unusual to find two on one side and three on the other.) The canines are massive cones, used for offensive purposes in the males of some species but basically the prey-catching tooth as in land carnivores. The molars are no longer grinders or mashers but consist of a single major cusp or spike with one or more smaller cusps on each side in line with the length of the jaw. In this way there is formed a long line of pointed cusps of varying heights admirably adapted to the retention of struggling prey. The only exceptions are the walrus and the bearded seal. The walrus feeds on large bivalve molluscs, like clams and mussels, which are crushed between stub-like molars. The bearded seal is not predominantly a fish feeder, shrimps, crabs, holothurians, clams, whelks, snails and octopus forming the bulk of its food and the molars are usually much worn and not sharply pointed. In some species of seal the young do have milk teeth for a short time. Usually there is only one set of functional teeth, the milk dentition being resorbed while the foetus is still in utero. Cetacea either have no teeth and feed by a filter mechanism (whale-bone whales) or have a single row of simple cusps (the toothed whales, porpoises and dolphins), all alike, which are far more numerous than in any other mammal. This is a high specialisation for fish-eating, much greater than the pinnipede condition. The Sirenia have flattened grinders of degenerate form since the seaweeds on which they feed hardly require munching.
The skulls of most pinnipedes show traces of slow and incomplete ossification. This is particularly true of the region on a level with the eyes so that the front part can be easily detached from the hinder brainbox in even old animals. The fur-seals and seals differ considerably in the general appearance of their skulls thus lending support to the view that they are derived from different sections of the land carnivores (Fig. 3). The walrus is again an exception since there is massive ossification to provide support for the huge tusks. Cetacea have evolved quite differently since many of the cranial bones contain spaces filled with air or occasionally with oil. The Sirenia have massive skulls although the bone itself is not dense.
We now come to the respiratory modifications and it is impossible to separate these from peculiarities of the blood system since the oxygen required by the tissues is transported by the haemoglobin in the red cells of the blood. The modifications of the nostrils have already been mentioned, but there are others equally significant. Many pinnipedes have cartilaginous rings in the trachea which are incomplete on the upper side but some, and both of our British species are among them, have complete rings which thus prevent any collapse of the trachea when the seal is under pressure in diving. These rings are continued into the bronchi and bronchioles and cartilage continues to be found