PHOLADACEJE J TEREDO OR WOOD-WORM. 417 



lines the cavity ; and thus a complete additional tube of shell is 

 formed, of a length proportional to the age of the animal.* At 

 the entrance of the tube, or the termination of the siphons, there 

 is a pair of pallettes, or small valves of shelly structure ; by the 

 motion of which the current of water is maintained, that is 

 necessary to bring a supply of food and oxygen to the animal thus 

 included. The highest point at which they commence to bore, 

 is always some feet below the lowest water-mark ; and they 

 usually work downwards. The orifice, being made when the 

 animal was young, is very small, and often difficult to perceive. 

 The beginning of the tube is usually horizontal or oblique ; and 

 afterwards it curves into a nearly vertical direction. The nature 

 of the wood has a great influence on the regularity and direction 

 of the canal which is hollowed in its interior ; but this is still 

 more affected by the neighbourhood of other Teredines, to avoid 

 whose tubes the animal will make sudden curves. It is difficult 

 to comprehend how it can become conscious of their proximity. 

 There can be no doubt that, in this instance, the excavation is 

 effected by the valves ; since it is always cylindrical, with very 

 smooth walls ; and the shell is adapted, both by its sides and 

 edges, for rasping and boring. 



1045. These animals are among the most formidable destroyers 

 of the works of Man. When it is considered that their instinct 

 prompts them to attack all the timber which he has, for various 

 objects, placed beneath the surface of the sea, it is seen that the 

 field of their operation is immense. The piles of bridges, piers, 

 and harbours, as well as shipping, are liable to their devastations ; 

 and Holland has been more than once threatened with an inun- 

 dation, by the destruction of the dykes which they have effected. 

 Many vessels have sprung . leaks and foundered, owing to 

 the unsuspected demolition, of the planking of their bottoms by 

 the same means. The most effectual protection against their 

 attacks, is a metallic sheathing : but it is said that piles may be 

 secured by the previous charring ot their surface to the depth of 

 a few lines. Different species of Teredo appear to exist in all 



The accessory pieces of the shell of the Pholas, may probably be considered 

 as a rudiment of the same structure ; they vary considerably in number and 

 amusement ; and sometimes appear to form the commencement of a regular tube. 



