PELECYPODA. 



37 



Fig. 25. 



The siphons are long in the Piddocks, united except near the end, 

 and enclosed in tough skin. The species are world-wide in their 

 distribution, and several are found fossil in some of the Tertiary 

 formations. 



The Teredinidcs, or Ship-worms, are also borers, like the Pholads, 

 but do not perforate rocks. They are 

 principally wood-borers ; the large Ku- 

 pltus arenaria, which is an exception, 

 living buried in the sand. The Ship- 

 worm has a long worm-like body, from 

 6 to 12 inches in length, which is more 

 or less enclosed in a thin shelly tube or 

 sheath. The true bivalved shell is at the 

 thicker end, and protects the mouth, 

 labial palps, the liver, and other internal 

 organs. At the opposite, or more slender, 

 end of the animal, the mantle is pro- 

 duced into two small tubes, one of which 

 conveys the water to the gills, whilst 

 through the other the water is expelled, 

 charged with the woody pulp excavated 

 by the foot. At the end there is a pair 

 of pallets, or paddles as they are some- 

 times termed, which are probably used as 

 a means of defence, in closing the shelly 

 tube after the contraction of the siphons. 



These animals are most destructive 

 to ships, piers, &c. ; and wood, which 

 is not protected by metal, when once 

 attacked, is soon riddled through and 

 through. They generally work with 

 the grain, and only turn aside when a 

 knot comes in their way ; and although 

 the holes may be all but touching, they 

 seldom appear to run into one another. 



The " Watering-pot shell " (Brechites) is a very remarkable 

 structure, and unlike the shell of an ordinary bivalved mollusk. 

 On looking carefully, however, near the perforated end (the 



Ship-worm 

 ( Teredo norvegica). 



a. Animal, removed from 



its shelly tube : p, p, 

 pallets ; s, exhalant 

 siphon ; a', inhalant 

 siphon. 



b, c. Different aspects of the 



shell. 



