RESPIRATION IN SHORE ANIMALS 209 



crabs do not enter the water but take up their position close 

 to the water-Hne and, with the appendages of one side firmly 

 anchored in the sand, wait till an extra high wave washes 

 over them ; they then return immediately to the higher parts 

 of the beach. Like other land-crabs Ocypoda breeds in the 

 sea and it will also take to the water if hard pressed by an 

 enemy. 



Respiration in Relation to Structure and Habit. — Certain 

 structural features of shore animals and certain of their 

 habits also have had considerable influence on the function 

 of respiration. Most molluscs, for instance, in consequence 

 of the presence of a shell and the concealed nature of the 

 branchiae, require some special means of taking water into 

 and expelling it from the branchial chamber. In most 

 herbivorous Gasteropods {Littorijia, Trochus) the water is 

 carried to and from the branchial chamber by a simple 

 extension of one of the mantle lobes, the incoming and 

 outgoing streams being separated by a flap of tissue hanging 

 from the lobe. In carnivorous molluscs, on the other hand, 

 a more or less completely closed tube — the branchial 

 siphon — has been developed from a fold of the mantle- 

 surface for the express purpose of conducting water to the 

 gills. The spent water does not return through the siphon, 

 but by means of the cilia situated on the gills is driven 

 towards the anus. Frequently, the siphon is protected 

 along the whole of its length by a special prolongation of 

 the shell called the canal, as in Murex ; in the whelk and 

 dog-whelk, however, the canal is simply a long notch in 

 the mouth of the shell. In the group of the Zygobranchiata, 

 which includes the key-hole limpet (Fissurella), common 

 limpet and " ormer " {Haliotis), there may be one or more 

 special apertures in the shell for the water to escape after 

 it has passed over the gills (see Cooke, 1895). 



A consideration of the siphons of bivalve molluscs is 

 best deferred until we come to deal with the question of 

 respiration in relation to the habit of burrowing. 



Other shore animals, besides molluscs, in which the 

 presence of a shell or, what comes to very much the same 



p 



