MOLLUSOA OF PLYMOUTH. 411 



weeds. It is often extremely difficult to distinguish a small 

 Aplysia when among red weeds. The colour of large Ajplysise is 

 generally that of the littoral Fuci, and of somewhat smaller speci- 

 mens that of Laminaria. It is perhaps significant that an Aplysia 

 migrating with growth from deep water to the shore would pass 

 through algae coloured first red, then brown, and finally olive-green. 

 These are the stages of its own colour-changes. 



NEW SERIES. — VOL. I, NO. IV. 31 



