THE GENERAL CHARACTERISTICS OF SHORE ANIMALS. 13 



give. These are only a few examples of partnership or 

 symbiosis, which is a common phenomenon among shore 

 animals. It is very apt to degenerate into parasitism, where 

 the one partner not only gets house room, but actually lives 

 upon the host. 



We have seen that the shell of shellfish affords an 

 apparently efficient protection against many dangers, but it 

 is important to note that not a few Univalves have entirely 

 lost their shells. These constitute the forms known as sea- 

 slugs, sea-lemons, and more generally as Nudibranchs, or 

 " naked-gilled " forms. Many of these occur on the shore, 

 and though on account of the absence of any means of 

 protection against drought they are confined to the zone 

 near low-tide mark, yet there they are abundant enough. 

 Many of them are very brightly coloured, and most are 

 furnished with little processes, either simple or branched, 

 which decorate the back, and add greatly to the beauty. 



FIG. <i.Doto coronata, a sea-slug with the back ornamented with curious 

 branched processes. After Alder and Hancock. 



In spite, however, of the frequent conspicuousness of the 

 animals, and the absence of any protective shell, there can 

 be no doubt that they are very rarely attacked or eaten by 

 the other shore animals. Many naturalists believe that the 

 bright colours and conspicuous processes are an advertisement 

 of inedibility, like the vivid colouring of some inedible 

 caterpillars. It is interesting to note, on the other hand, 

 that while many Nudibranchs are conspicuous and highly 

 coloured, others are exceedingly like, the weeds and corallines 

 among which they live. Thus Doto coronata (Fig. 4), a 

 beautiful and not uncommon sea-slug, is very like the 

 common coralline, or pink limy weed, and is exceedingly 



