I2 4 



Descriptive Zoology. 



wood, it soon dies. The hole by which the larva enters the wood is 

 hardly larger than a pin head, but as the animal grows it excavates a 

 constantly widening tube, thus imprisoning itself for life. Just how it 

 burrows is not certainly known. It does not feed upon the wood, the 

 fine sawdust being carried off through the excurrent siphon. Its food 

 consists of microscopic plants and animals, which are brought in by 



FIG. 76. RAZOR SHELL CLAM. 



currents, as in the clam, and its only communication with the outer 

 world is through the small hole by which it first entered the wood. 

 Shipworms work rapidly, often completely honeycombing the wood. 

 But no matter how many of them there are in the wood, their tubes 

 never interfere with one another, but there is always left a thin partition 

 between. They avoid iron rust, so timbers are protected by driving 

 them thickly with broad-headed nails. The copper sheathing of hulls 

 of ships is the best protection. Shipworms caused the famous dam 

 break in Holland at the beginning of the last century. 



The Razor Shell Clam. The razor shell clam has a shell somewhat 

 resembling in shape and size the handle of a razor. The foot projects 



at the anterior end, the siphons 

 at the posterior end. These 

 clams make vertical holes in 

 the sand and can dig rapidly. 

 At low tide the posterior end 

 may be seen projecting from 

 the sand, but unless the col- 

 lector approaches quietly and 

 seizes the clam quickly, it is 



FIG. 77. MUSSEL. 



With threads by which it is attached. 



almost sure to escape. They 

 seem to be very sensitive to 

 vibrations, and probably be- 

 come aware of approach through these rather than through hearing or 

 sight, although they are somewhat sensitive to light. 



The Salt-water Mussel. One of the most common marine bivalves 

 is the mussel. The shell is usually dark or purplish, and rather thin 



