CH. II] THE CRAYFISH 69 



place at the junction of the first and second joints. At 

 this spot a double diaphragm exists with a single central 

 hole through which pass nerves and blood vessels. When 

 the limb is cast off the hole, being very small, is soon 

 stopped up by coagulated blood. From the stump a small 

 new limb is developed, which, after a few moults, gains the 

 normal size. The device is clearly for the protection of 

 the crayfish : an enemy may seize a limb and keep it, 

 but the owner escapes and is very little the worse. 



Foes and Parasites. Crayfish fall victims to many 

 birds of the Duck tribe, and to herons, who split them 

 open by hard blows on the back. They are also caught 

 and eaten by water-voles, being then dragged ashore and 

 consumed on the bank. 



Their parasites are not very noticeable. A very large 

 Gregarine Protozoon, Porospora gigantea, is found in the 

 alimentary canal. On continental crayfish, which are more 

 often procurable from dealers than our English species, 

 there occurs among the eggs and on the swimmerets 

 of the female the remarkable leech (or Oligochsete worm ?) 

 Astacobdella or Branchiobdella. This leech is about 

 J inch long, and has a well-developed body cavity divided 

 by transverse partitions. Its eggs, which are more often 

 to be seen than the adult creature, are laid singly and 

 attached by a slender stalk to the swimmerets of Astacus. 

 The parasite is charitably credited with devouring only the 

 addled eggs of the crayfish, and thus acting the part of 

 a useful scavenger. 



Encysted upon the surface of the intestine, and else- 

 where within the body, there are frequently to be found 



