350 THE BIOLOGY OF THE SEASONS 



the littoral economy by cleaning things up. They devour 

 everything that is edible in the dead bodies of animals, 

 both large and small, and make beautifully clean skeletons, 

 just as the ants do in the meadow. Along with these 

 genuine inhabitants of the shore there are representatives 

 of the same crustacean class in the jetsam. In the early 

 Summer especially we find many moults from the glassy 

 husks of acorn-shells or barnacles to the substantial cast 

 shell of the edible crab. 



On many beaches the most conspicuous component of the 

 jetsam is that furnished by molluscs, whose shells afford an 

 unstinted " beauty-feast." In shape and colour they are 

 singularly attractive, and they are full of unsolved problems. 

 Even to the simplest question, What is the chemistry of 

 their formation ? we can give no answer, though there are 

 reasons for suspecting that shell-making is an organised way 

 of using up waste-products. 



In the regular lines patent on the surface of the shell, 

 and often accentuated by fine gradations of colour, we 

 have a fine instance of the periodicity of growth, and 

 doubtless also, if we knew enough, of punctuation from 

 without. 



To get a good instance of the correlation of habit and 

 structure, we have only to lift a dog- whelk's shell and contrast 

 it with a periwinkle's. The former has a notch at the mouth 

 of the shell, in which, during life, there lies a breathing-tube 

 (or respiratory siphon) ; the former has no notch and no 

 siphon. And the interesting point is, that those with the 

 notch are almost invariably carnivorous and those without 

 the notch are vegetarian. 



Many of the bivalves, such as the very common Venus 

 gallina, show a neatly bored hole up near the hinge, and 

 this explains their presence in the jetsam ! The hole was 

 bored by a carnivorous gastropod, which killed the bivalve 



