METHODS OF ATTACK AND DEFENCE 109 



The best example of weapons with a purely mechanical 

 action is furnished by the chelae of many of the higher 

 Crustacea (crabs and lobsters). The appearance and mode 

 of operation of these are too well known to need description. 

 An interesting fact, however, in the lobster, is the almost 

 invariable differentiation of the chelipeds into a large 

 crushing pincer and a smaller cutting one. The latter kind 

 is more slender in form with numerous accurately fitting 

 teeth along the edges of the blades, while the crushing claw 

 is heavier in appearance, and the teeth are large, blunt, and 

 irregular. The lobster has been seen to hold its prey with 

 the cutting forceps, and at the same time to administer a 

 formidable blow with the larger claw. Indeed, it has been 

 the experience of one of the writers to be treated in this 

 way. Lobsters may be right-handed or left-handed, the 

 large crushing claw occurring on either side. Occasionally 

 specimens occur with the cutting forceps on both 

 chelipeds, and very rarely with crushing claws on both 

 sides. 



The strength of Crustacean chelae is often enormous. 

 Plateau (quoted by Fredericq, op. cit.) has shown experi- 

 mentally that the left chela of Carctnus moenas is capable of 

 supporting a weight of nearly two kilograms, which is 

 equivalent to nearly thirty times the weight of the body, 

 while the right chela is still more powerful. Compare 

 this with the average grip of a man's right hand, which, as 

 tested by the dynamometer, is only capable of exerting a 

 pull of 50 kgs., or a little more than two-thirds of his own 

 weight. 



It is not out of place here to speak of the strength of 

 the adductor muscles of the molluscan shell, as it is largely 

 on this that the safety of these otherwise defenceless animals 

 depends. In estimating the power of contraction of the 

 muscles of various common bivalves Plateau takes not a unit 

 weight of muscle, but a unit sectional area of one square 

 centimetre, as in this way complications due to the number 

 and the length of fibres are avoided. 



