210 THE SEAS 



they seize with their powerful teeth. Seals and sea-lions 

 feed on fish which they pursue and capture in large numbers, 

 but walruses live on shellfish which they scrape up by means 

 of their large tusks. 



Suckers and Parasites 



The only type of feeders which we have not yet con- 

 sidered are those which live by sucking in fluids or soft 

 tissues. These are not numerous in the sea except as 

 parasites, although a few animals such as some of the little 

 nudibranch molluscs or sea slugs which live on green weed 

 are suckers without being parasitic, merely drawing in the 

 soft green tissues. There are a great many parisites 

 which do not dwell within the body of the animal on which 

 they are parasitic, but either fix themselves permanently 

 or move about on the outside of the body. These are 

 known as ectoparasites to distinguish them from the 

 endoparasites which live within the body of the host. 

 There are many examples of ectoparasites in the marine 

 world, the commonest being copepod Crustacea, relatives 

 of the freely swimming copepods which abound in countless 

 numbers in the sea, forming, as we have seen, the principal 

 food of fish such as mackerel and herring. These are usually 

 parasitic on fish and assume all manner of strange shapes; 

 a few of them can be recognized as copepods but in the 

 majority it is only a knowledge of their life-history in the 

 early stages which has enabled us to classify them as 

 copepods. Examples of different types of these parasites 

 are given in Plate 81. Some of the less degenerate kinds 

 move about freely on the surface of their host fish, probably 

 living on the mucus and soft skin, but the more degenerate 

 ones bore into the tissues of their host or fix themselves 

 to the gills, or soft skin round the eyes, and have usually 



