138 



THE SEAS 



down a long process known as the apophysis. The two 

 halves of the shell are attached by the two knobs so that 

 the valves are able to rock backwards and forwards upon 

 these two points, first the hinder and then the frontal 

 regions coming closer to one another. 



This rocking process takes place regularly in life, the 

 motive power being supplied by tw^o pairs of muscles, 

 one of which runs between the hinder regions of the shell 

 and the other between the frontal resfions. From between 



ci.CL 



CL.CUI. 



p.CLci. 



v.cc. 



Fig. 29. — Shell of ShipA-orm (Teredo). Inner and outer views. 



a.au., ana t.aa., atiachments of muscles; ap., apophysis; 



a.l., tn.l., and p.l., parts of shell ; d.a. and v.a., articulating knob. 



the valves in front there projects a Httle round sucker, 

 which corresponds to the " foot " or organ of movement 

 in such animals as the cockle. The muscles which work 

 the foot are attached to the apophysis and by their con- 

 tractions enable it to grip the wood at the head of the 

 burrow as shown in Figure 28. At the same time a flap of 

 skin which overlaps the shell above presses the animal firmly 

 against the wood in that region. With the shell pressed 

 tightly against the head of the burrow in this manner it is 

 easy to see how boring takes place. By the contraction of 



