124 Mr Thompson on the Teredo navalis • 



subject it is remarked in " Rees's Cyclopaedia," (copied, I be- 

 lieve, from the " Historia Naluralis Teredines'" of Sellius), that 

 " the kind of wood in which these worms are lodged, makes a 

 great difference in the appearance of their cells, as they work 

 much more speedily and successfully in some kinds than in 

 others. The fir and alder are the two kinds they seem to eat 

 with the greatest ease, and in which they grow to the greatest 

 size ;" but in this same article their " utmost bigness ' is said to 

 be " a foot."" 



Though it is well known that the Teredo l)ores in the direc- 

 tion of the grain, it ma}' be observed that it does so whether the 

 position of the wood be perpendicular or otherwise. Captain 

 Fayrcr indeed remarks, that it " has a decided disposition to 

 work horizontally.''' It is, however, often obliged to deviate 

 from a straightforward course, and exert its power of working 

 against the grain to avoid such bbstructions as nails, timber- 

 knots, and the tubes of its fellows, and make a winding or an- 

 gular habitation, accordingly as such impediments occur ; but 

 these circumstances seem not eventually to impede the progress 

 of the animal, or the formation of ils shell, as some of the very 

 largest specimens I have examined are the most tortuous. 



Captain Fayrer states, that at Portpatrick the Teredo does not 

 seem to have any preference to situations that are occasionally 

 left dry (See Osier in Phil. Trans, v. cxii. p. 358), as it is found, 

 taking medium for tides, at the depth of thirty feet, but that he 

 has nevertheless obtained the largest specimens near the surface. 



As I have never had an opportunity of seeing the animal Teredo 

 navalis alive, I am only enabled, from an examination of speci- 

 mens recently dead or preserved in spirits, and of timber sub- 

 jected to its operations, to offer an opinion on that part of its 

 economy, above all others the most interesting — the method by 

 which it forms its cell, and thereby causes such vast destruction 

 to shipping, and to timber exposed to its influence in the sea. 

 On this subject, authors of eminence are directly opposed to each 

 other in opinion. Sir Everard Home and Mr Osier attribute 

 the perforation to the unassisted agency of the primary valves ; 

 whilst Colonel Montagu and Dr Turtoa* as firmly believe that 



* The climax of the argument used by Dr Turton is, that " the internal 

 lerniination of every pei-foration is spherically concave, and not abniptly trun- 



