lyo THE BIOLOGY OF THE SEA-SHORE 



being added along the edges of the notch. The attachment 

 and consequently the movement of the valves is very different 

 from that of the ordinary Lamellibranch shells. The hinge 

 ligament is either absent or vestigial and the hinge itself is 

 represented by a prominent knob on each valve which works 

 against its fellow. At the ventral edge of each shell is another 

 knob which forms a second point of contact between the 

 valves. Next, the two adductor muscles, instead of con- 

 tracting simultaneously, do so alternately, drawing together 

 now the front and now the hinder edges of the valves. In 

 this way a see-saw like movement of the valves is produced 

 about the pivot formed by the dorsal and ventral knobs, 

 which causes the tiny teeth to rasp against the wood. In 

 addition, it is probable that the action of other muscles, 

 attached to a narrow curved process projecting from a 

 point close to the dorsal knob, gives rise to a twisting move- 

 ment of the whole shell which aids progress. Purchase is 

 obtained by the adhesion of the sucker-like foot which 

 projects between the notches in the valves. 



A related form, Xylophaga^ is recorded by Thompson 

 as destroying, in company with other forms, the submerged 

 woodwork in the harbour of Ardrossan on the Firth of 

 Clyde (see Plate XII). 



Certain Isopod and Amphipod Crustaceans are also 

 found excavating galleries in wood, the process here being 

 simply one of gnawing. Of these the most important on 

 British coasts is the " Gribble," Limnoria lignorum, an Isopod, 

 which may occur near high-tide mark but is said to be most 

 abundant between low-water and half-tide mark. The 

 burrows formed by this species are about ^th of an inch 

 wide and of uniform diameter throughout. They run 

 obliquely through the wood and, in consequence, though 

 from 1 1 to 2 inches long, the average depth is usually not 

 more than about half an inch (Caiman, op. cit.). 



Associated with Limnoria there frequently occurs the 

 Amphipod Chelura terebrans. Owing to the somewhat 

 larger size of this species the burrows made by it are a little 

 wider than those of Limnoria ^ reaching yo^^ ^^ ^^ ^'^^^ ^^ 



