1 40 THE SEAS 



amount of nourishment from the wood, the fragments 

 of which are acted upon by digestive juices, and sugars 

 produced which can be absorbed by the tissues. There is, 

 moreover, a large extension of the stomach which is always 

 filled with shavings of wood and seems to be well adapted 

 for the storage of such a slowly -digested substance. 



Once encased in its burrow no Shipworm can ever leave 

 it. As we have seen the opening of the burrow is not 

 much larger than a pinhead whereas, within, the burrow 

 widens out quickly and may, in the common species, be 

 some half an inch in width. Moreover, the animal is actually 

 attached to the edge of its burrow in the region of the 

 pallets. If a Shipworm is taken out of its burrow, no matter 

 how carefully this is done, the animal cannot make a new 

 burrow for itself. Since this is the case how does it happen 

 that new wood becomes infected, as it does very quickly 

 if conditions are favourable for the growth of the Ship- 

 worm ? During the spring and summer especially, the 

 eggs and sperms are discharged in immense numbers 

 into the sea by way of the second of the siphons. There 

 the young Shipworms develop and at this early stage of 

 their existence they are exactly like young mussels or 

 similar bivalves. After a short time a pair of tiny shell 

 valves appear which entirely enclose the body and from 

 between which a crown of small hairs or " cilia " can be 

 protruded by means of which the little animals are able 

 to swim about, in exactly the same manner as the oyster 

 (Fig. 59). This freely swimming ' larval " Shipworm is 

 shown in Figure 30. It is not known for exactly how long 

 the Shipworms remain in this state, but they can probably 

 do so for several weeks, and during this time they may be 

 carried for great distances by ocean currents or wind drifts. 

 This early stage in their existence is the only time when the 

 Shipworms are able to move about freely in the sea and 



