270 Shell Life 



spreads up from the head to cover the base of the 

 shell and so protect it from destruction by the sand 

 through which the creature burrows ; others swim in 

 the open waters by means of their foot-expansions ; 

 but most of them crawl over seaweeds, sponges, or 

 corallines, much after the manner of the land-slugs. 

 Some of them feed upon seaweeds and then agree 

 with these in colour, whether green, olive, or red ; 

 but many devour living sponges, sea-squirts, and 

 zoophytes, and then often reproduce in the decora- 

 tion of their own bodies the pattern and colour of 

 the surface upon which they are at once feeding and 

 being hidden. 



The order Opisthobranchiata is divided into three 

 sub-orders, each divided again into sections, families, 

 and genera. We cannot go into all these divisions, 

 but we will glance at representative species from 

 each of the families found in British waters. The 

 first sub-order is the Tectibranchiata, or those whose 

 gill-plume is covered by the folds of the mantle. 

 They possess a shell, which is more or less enveloped 

 in folds of the mantle or foot, and is often rudi- 

 mentary. Some of the families included in the first 

 section {Bulloklea) appear to have no claim to be 

 reckoned among Sea-slugs ; but these species, other- 

 wise closely allied to those whose shells have vanished, 

 must be included, and they indicate the probable 

 evolutionary course travelled by the entirely shell- 

 less kinds. Take the Acta3on-shell {Ackeon tornatilis) 

 to begin with. The oval shell is sufficiently solid in 

 structure for its neatness and regularity to have 

 suggested to Linnaeus that it had been turned in a 

 lathe, so he called it tornatilis to express that idea. 



