210 HALF AN HOUR WITH SHELL-FISH (BIVALVES). 



just protruding above. These are usually double, 

 one being for the purpose of admitting fresh sea- 

 water, and the other for excretory purposes. The 

 water thus admitted not only brings food to the 

 animal, but aerates its gills with oxygen as well. 

 The sexes are usually distinct in bivalves, and there 

 is occasionally a slight difference in the shells to 

 indicate them. Thus, in the oyster, the male shell 

 generally has the umbone, or plate near the beak, 

 white and humpy a fact worth knowing to oyster- 

 eaters who don't care about " green " natives. 



There is no doubt that the commonest objects 

 along our coasts are the cockles and mussels and a 

 good thing for us that such is the case. The former 

 is too well known to need description, but we have 

 other species, some of which are much larger and 

 handsomer than the edible one. In all, the foot of 

 the living animal is very brilliantly coloured, 

 generally with vermilion. The mussels have an 

 enormous power of increase, or they could never 

 withstand the ravages made on them. In deeper 

 water we have a kind popularly called " Horse- 

 mussels " (Modiola), whose detached shells may 

 often be gathered after a storm. One species 

 belonging to this is really very^ pretty, and is 

 called M. tulipa, on account of the part about the 

 beak being coloured with streaks of crimson or 

 violet, like the painted petals of a tulip. Associated 

 with the above objects, the visitor will scarcely fail 

 to find the more brittle valves of the " Eazor-shell," 



