EVOLUTION AND EXTERNAL APPEARANCE 



89 



Figure 4/. Acorn Barnacles and Stalked Barnacles on the skin of a Humpback. 

 Photograph : A. F. M. Drieman, Amsterdam. 



on their hosts, bvit merely attach themselves to their skins without doing 

 any damage. Now the smooth skin of fast swimming animals does not 

 really provide safe anchorage for parasites, and for that reason by far 

 the overwhelming majority of these guests entrench themselves firmly in 

 the epidermis and even in the blubber beneath. With the exception of one 

 encapsuled representative of the unicellular Ciliate, Haematophagus by 

 name (cf. Chapter 10), and of a Nematod {Odontobius) , which both live on 

 the whalebone, all these guests are Crustaceans, first appearances not- 

 withstanding. Unlike the external parasites of terrestrial animals (e.g. 

 fleas and lice) all of which are air-breathing insects (with the exception 

 of mites and ticks which are classified with spiders), aquatic parasites 

 naturally breathe in water and must therefore be equipped with gills or 

 similar organs. 



The most striking of these guests are the Acorn Barnacles, sessile 

 crustaceans with a hard shell of calcium, which most of us have seen 



