CRUSTACEA 53 



From what has been said, it is apparent that a crayfish 

 is well able to find substances fit for food and to reject that 

 which is unfit. Elaborate and specialized systems of 

 organs for digestion, assimilation, circulation, respiration, 

 and excretion are present. 



Self -protection. The solitary crayfish has many ene- 

 mies fish which seek to devour it, parasites which suck 

 its blood or sap its vitality by infesting the interior of the 

 body, minute moulds or bacteria which cause diseased 

 conditions. How does it escape these? 



Larger enemies are usually avoided by hiding in crevices 

 between stones or water plants. If a crayfish is dislodged 

 from its retreat, it swims quickly backward by flipping the 

 powerful tail fin and often finds a new hiding place (Fig. 29). 

 But if captured, it does not surrender without a struggle. 

 The great claws inflict painful wounds and the smaller legs 

 scratch the cornered animal fights literally " tooth and toe- 

 nail," until it is bested or escapes. Even if a leg or two is 

 lost the damage is not serious for a new one is formed 

 beneath the exoskeleton, and after the next moult is nearly 

 as large as before. On account of the large spaces within 

 the body, a crayfish easily bleeds to death if the body wall 

 is punctured. In the appendages, however, excessive 

 bleeding is prevented by a special arrangement. A crushed 

 leg is thrown off at a " casting joint" near its base, where 

 there is a very small aperture from the blood sinuses and 

 the opening quickly closes. 



The color of a crayfish is usually more or less like the 

 surroundings, and can be changed somewhat by the con- 

 traction or expansion of little pigment cells in the skin. 

 Such " protective resemblance" makes it less conspicuous. 



The insidious enemies which attack the crayfish as para- 

 sites are warded A off chiefly by sanitary measures. Scrupu- 

 lous cleanliness is observed. There are special combs and 

 bristles on the appendages for cleaning the exoskeleton 

 and it is continually 'scraped and polished. If there is a 

 break in the outside covering, the way is opened for various 



