EROSION OF SHELLS. 215 



statttly being carried down towards the shells by the water, 

 attach themselves to the most prominent portion, which is at 

 the same time the oldest and has the thinnest cuticle ; they 

 gradually penetrate the shell, and thus the calcareous layers are 

 exposed to the action of the water ; this, in consequence of tho 

 carbonic acid it contains, eats into the prismatic layer, and, as 

 a minute vortex must be established in each little hole by the 

 action of tho current, the chemical eflect may be enhanced by 

 the mechanical action of the stream. By degrees this erosion 

 proceeds more rapidly than the destruction of the cuticle by 

 the action of the fungus, and thus long strips or i-ags remain 

 free, covering the pits worn by the water. The pits at last 

 show to some extent unmistakable traces of the chiselling action 

 of minute whirlpools. 



Of course, in time, these will gradually eat through the 

 nacreous as well as the prismatic layer, even without the 

 assistance of the fungus, and finally the soft portion of the 

 animal itself will be laid bare. The creature protects itself 

 against these injurious effects simply by secreting fresh layers 

 of calcareous matter, and thus the structure of the shell 

 is considerably altered. It is certainly difficult to under- 

 stand how the creature is able to secrete a new supply of 

 calcareous matter precisely at the spot where the shell grows 

 thin; for this does not take place in the first instance only 

 when the shell is actually worn through, but without exception, 

 on the contrary, at a much eai-lier stage; as is proved by the fact 

 that shells pierced quite through are never, or extremely seldom, 

 found. It may perhaps be assumed that the impact or pressure 

 of the whirlpool is more perceptible at the thinner portions of the 

 shell to the creature within, until at length the local irritation 

 it produces excites a more copious secretion of the shell-forming 

 fluid by the skin. Thus the same power which is exerted to 

 destroy the shell at the same time incites the animal to defend 

 itself against its injurious effects. 



Similar results from currents on the animals living exposed 

 to them, whether free and creeping or occasionally sedentary, 

 could be pointed out in many other cases ; thus, for instance, 

 the forms of the shells of many univalves seem especially 



