212 THE BIOLOGY OF THE SEA-SHORE 



has shown that in autumn certain species of worms and 

 crustaceans may begin to suffer from a kind of poisoning 

 due to the over-accumulation of toxic products in the sand. 

 On the other hand, the same writer shows that water in 

 which certain seaweeds have been Uving (red seaweeds and 

 Lithothamnion in particular) tends to become very toxic to 

 animal life on account of its excessive alkalinity, but filtering 

 through sand rectifies this. In a sense, therefore, the sand 

 offers a chemical as well as a physical protection to the 

 animals which frequent it. 



In bivalve molluscs, which are permanent burrowers 

 in a stricter sense even than worms, the gills lie in a large 

 cavity — the mantle chamber — which is placed in communica- 

 tion with the water above by means of two tubes or "siphons." 

 Of these, one serves to conduct the water in, the other to lead 

 it out again, after it has fulfilled its purpose. The siphons 

 may be separate for practically the whole of their length or 

 may be more or less completely united. The length of the 

 siphons is, with few exceptions, an accurate index of the 

 depth to which the animal burrows. The circulation of 

 water through the mantle cavity is ensured by the cilia of 

 the gills. Actually, this ciliary current serves, as in a number 

 of other cases, the dual purpose of aerating the animal and 

 conveying to it its food. The term " gill " is really a 

 misnomer in this case, the so-called gills being nothing more 

 or less than food-collecting organs ; the true organ of 

 respiration in bivalves is the mantle. Nevertheless, it is 

 obvious that the siphons constitute an adaptation to subter- 

 ranean breathing as much as to feeding. 



Several of the larger Crustaceans spend a good deal of 

 their time underground and show very distinct respiratory 

 modifications in consequence. Both in the common shore 

 crab and in the swimming crabs, the claws together with the 

 marginal spines form a sieve through which the water of 

 respiration is filtered free of sand. In Calappa and others 

 which have no filtering spines there are crest-like expansions, 

 divided into spines and notches, on the upper side of the 

 claws themselves (Garstang, 1905). The greatest amount of 



