142 THE SEAS 



dication of this fact ; finally, however, it crumbles away 

 (Plate 53). Shipworms do not live long, a year or perhaps 

 two years. When they reach a certain age or when there 

 is no further wood to bore into, they continue the shelly 

 casing over the front end of the burrow and remain 

 quiescent within this, taking such food as they can obtain 

 from the water, until they die. 



The action of the Shipworm in our own waters is much 

 slower than that of the tropical species. Here, a piece of 

 untreated wood is seldom attacked until it has been in 

 the water for at least a year, whereas in the South Seas 

 wood will become infected in a few weeks and after six 

 weeks show a similar degree of infection to wood which 

 has been exposed here for eighteen months. 



There are many different species of Shipworms but they 

 all belong to two genera, one called Teredo and the other 

 Bankia. All the British Shipworms belong to the genus 

 Teredo, of which three species are found in our seas, and 

 none of them construct tubes longer than about eighteen 

 inches, the majority much less, but there are tropical 

 Shipworms which are much larger, one, the " giant Teredo," 

 being reported to attain a length of two yards and become 

 as thick as a man's arm ! The members of the genus 

 Bankia are inhabitants of the tropics and are easily dis- 

 tinguished from Teredo by the structure of the pallets, 

 which are paddle-shaped in Teredo but are long and feather- 

 like in Bankia. 



There are two other bivalve molluscs which live in wood. 

 One, known as Xylophaga, is commonly found in floating 

 timber in temperate seas, it has a shell very like that of 

 Teredo and it bores in the same manner, but there is no 

 elongate body, the siphons project from the hind end of the 

 shell which completely encloses the body (Plate 52). The 

 burrows are short, seldom more than one and a half inches 



