MOLLUSCS. 



95 



V' 



it may seem all firm and sound, yet it is in reality but a shell, the worms 

 having honeycombed it in every direction. 



It is a peculiar feature of animal pests that they seem to have no 

 gradual growth into prominence, but seem to spring 

 into existence at once. Of this we shall have nu- 

 merous examples — the buffalo-bug, Colorado beetle, 

 and many others — before we finish this volume. It 

 is true of the form before us. All of a sudden in 

 1731—32 it appeared, and caused great damage to 

 the wharves and shipping of Holland. A government com- 

 mission was appointed to study its habits and check its rav- 

 ages, but it was not very successful. Gradually mankind dis- 

 covered methods of checking or circumventing the ship-worm, 

 and to-day it is of far less importance than it was a century 

 ago. Iron ships and copper sheathing on wooden ships are 

 beyond its powers. Still, it has the wharves and smaller boats 

 still left as its prey. Soaking with creosote is of but slight 

 avail, but iron rust seems to be distasteful to them, and so 

 timber covered with broad-headed nails seems tolerably secure. 

 Paint containing copper or verdigris is also, to a degree, repel- 

 lent to the ship-worm. 



The piddock (Pholas), though greatly different in external 

 appearance, is really a close ally of the Teredo. Like it, it is 



Fig. 87. — Piddock-shell (Pholas) in its burrow. 



Fig. 86. — Adult ship- 

 worm (Teredo nor- 

 vegica) taken from 

 its burrow: natural 

 size. The arrows at 

 the upper end show 

 the directions of the 

 siphonal currents. 

 The valves of the 

 shell are shown at 

 the lower end. 



