176 THE ANIMALS AND MAN 



young teredo is a free-swimming ciliated embryo like the 

 young of the other bivalve molluscs, but it soon settles on a 

 piece of submerged wood, usually the pile of a wharf, or the 

 bottom of a ship, and burrows into this wood. As it grows 

 it enlarges and deepens its tube-like burrow, and lines it with 

 a calcareous deposit. The burrow may be a foot long or 

 longer, and when thousands of teredos attack a pile or the 

 bottom of a ship, the wood soon becomes riddled with holes. 



Fig. 87. The giant yellow slug of California, Ariolimax californica. This 

 slug reaches a length of twelve inches. (From life.) 



These boring molluscs do great damage to wharves and 

 ships. In Holland where they were first discovered they 

 caused such injuries to the piles and other submerged wood 

 which supported the dikes and sea-wa,lls that they seriously 

 threatened the safety of the country. 



Perhaps one-half of all the known species of molluscs are 

 snails and slugs (fig. 87). Snails are either aquatic or ter- 

 restial in habit, but in either case they (the true pulmonate 

 snails) breathe not by means of gills, as do most of the other 

 molluscs, but by means of a so-called "lung." This lung is 

 a sac with an external opening on the right side of the body 

 and with its inner surface richly furnished with fine blood- 

 vessels. The exchange of gases between the blood and the 

 outer air takes place through the thin walls of the blood- 

 vessels. Most snails which live in the water, as the pond- 

 snails and the river-snails, have to come occasionally to the 

 surface to breathe. These fresh-water and land-molluscs 



