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The morphology and the congenital pathology of the heart of the common seal (Phoca vitulina vitulina) are described. Particular attention is drawn to the dorso-ventral flattening of the heart and the S-like course of the aortic arch. The following congenital malformations of th heart were discussed: (1) patent ductus arteriosus associated with a patent foramen ovale secundum and malposition of the entrance of the caudal caval vein: (2) aplasia of the origin of the left coronary artery: (3) coarctation of the aorta; (4) myocardial bridging. From this study of the heart of the common seal it is concluded that the resistance of the common seal to physical stress is poor. Survival of this animal in the Waddensea requires a quiet and undisturbed biotope.

The findings in seals submitted to the Research Institute for Nature Management, Arnhem, for post-mortem examination during the period from 1960 to 1981 are reviewed with reference to three tables. A striking feature was the large proportion (55 per cent) of young seals, well over 38 percent of which died in the first two months of life. There also was a large proportion of young seals and yearlings with parasitic infections, which would appear to suggest that it is difficult for the young animals to build up a good condition. This could be due to a disturbance during the nursing period and inadequate food of satisfactory quality. Of the grey seals fond on the coast of the Netherlands, 35.5 per cent were young and starved to death. Drowning in nets and hoopnets occurred in the young as well as in the older seals and both in common and grey seals, although the majority of the victims were yearlings.

The number of seals in the coastal waters of the Netherlands has been considerably reduced since 1950: in the Wadden Sea, it decreased from 3,000 to 500, in the Delta area, from 1,500 to merely a few animals. Studies on population dynamics during the period from 1974 to 1978 showed that reproduction among the seal population in the Netherlands Wadden Sea is too small compared with the stable population of Schleswig-Holstein. Investigations on the role of environmental factors in diminished reproduction centered on the factor water pollution. Analysis of the various contaminants in the tissues of seals of the Netherlands. Schleswig-Holstein and Denmark, which were found dead, showed tha particularly the concentrations of polychlorinated biphenyls in seals of the Netherlands were significantly higher than they were in German and Danish animals. The epidemiological and experimental findings on the characteristics of these contaminants, in conjunction with the high concentrations of polychlorinated biphenyls and the decrease in reproduction of seals in the Netherlands, led to the hypothesis that polychlorinated biphenyls are the cause of diminished fertility of seals in the Netherlands.

The common seal Phoca vitulina belongs to the order Pinnipedia, a group of marine mammals, the majority of which live in the colder seas. Two main groups are differentiated within this order: Otarioidea and Phocoidea. The Otarioidea are divided into two families: Otariidae (sea lions and fur seals) and Odobenidae (walruses): the Phocoidea consist of the family Phocidae (seals) only. The Pinnipedia are closely related to the order Carnivora: they probably originated about 50 million years ago from a group of mammals related to bears. Otarioidea and Phocoidea are now usually regarded as close relatives, n view of among others, biochemical and parasitological characters.