INVERTEBRATE BOTTOM FAUNA 



461 



position of the fauna of the different " facies " depends to a great 

 extent on the structure of the animal-forms, inasmuch as some 

 forms must have a vegetable or hard solid foundation, while 

 others require loose material. Littoral gasteropods, as a rule, 

 require a solid foundation, and they are therefore generally absent 

 from the sandy bottom ; but there are certain burrowing forms 

 which can only live where the bottom is incoherent. Other 

 forms, again, like the crab, are able to live on nearly every 

 kind of bottom. 



Fig. 315. 

 Balaiius ba/anoidcs, L. 



Below the low-tide area, with its fucus vegetation, we find 

 on hard bottom a Laminaria belt beginning immediately below 

 the fucoid belt, and always covered by water.^ We find also a 

 Zostera belt, hard bottom, and sandy bottom. 



On the bare or scantily overgrown rocks near high-water Low-tide 

 mark we find a white belt of barnacles {^Balamis balanoides, see ^'^'^^• 

 Fig. 315); when examined at high tide we notice these little 

 creatures extending and contracting their lash-like limbs to set 



^ Only at very low ebb-tides and in certain places do we find certain species of Lamiuaria 

 also laid bare. 



