ANIMAL LIFE-HISTORIES 



the shell, the front part of which is raised into a system of inter- 

 secting ridges with sharp projections. These are brought into 

 action by twisting movements on the long axis, during which the 

 flat surface of the cylindrical foot holds on to the end of the 

 burrow. Chemical action is excluded by the fact that piddocks 



are able to bore into hard siliceous 

 rocks as well as into those of cal- 

 careous nature. In spite of its use 

 for boring purposes the shell is quite 

 thin, probably because it has not 

 the same value for protective pur- 

 poses as in non- burro wing forms. 

 The later part of the life-history of 

 a form (Pholadidea) closely related 

 to the Piddock is of very great in- 

 terest. To begin with, it bores into 

 rock after the fashion of its relative, 

 but when full grown loses its foot, 

 closes up the front end of the shell, 

 and remains stationary for the rest 

 of its life. The hinder end of the 

 shell is produced into a sort of hol- 

 low cylinder, directed towards the 

 opening of the burrow, and enclos- 

 ing the two tubes (siphons), through 

 one of which currents bearing food 

 and oxygen continually flow, while 

 the other gives exit to return cur- 

 rents that carry off the various pro- 

 ducts of waste, and also enable the 

 eggs to make their way to the ex- 

 terior. Another form (Jouannetia\ 

 allied to the foregoing, bores into 

 a block of coral, excavating a spherical cavity exactly corres- 

 ponding to the shape of its body. When adult, the front of 

 the shell is closed up, and the foot lost as in the form last men- 

 tioned. If the shell were perfectly round it is clear that there 

 would be a danger of the animal slipping round so that the open- 

 ings of the siphons were directed away from the exterior, the 

 result of which would be fatal, as there would be no possibility of 



Fig. 933. Ship-Worms (Teredo navalis}. A 

 Bristle-Worm has appropriated an empty bur- 

 row on the left. 



