MOLLUSC/1: THE MOLLUSCS 253 



second or hinder pair), and the respiratory pore. Note the streak 

 of mucus left by the slugs in crawling about. 



Some sea-shells can be got from private collections of " curios" 

 to illustrate the variety of form of the univalve shells. 



Perhaps one-half of all the known species of molluscs 

 are snails and slugs (fig. 108). Snails are either aquatic or 

 terrestrial in habit, but in either case they (the true pulmo- 

 nate 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 sutjace 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 which possess a lung-sac instead 

 of gills constitute the order Pulmonata. The pulmonate 

 pond- and land-snails and slugs are vegetable feeders and 

 where they occur in large numbers do much injury to 

 vegetation. While the common pond-snails have but 

 one pair of feelers, at the base of which are found the 

 eyes, most of the land-snails and slugs have two pairs of 

 ' * horns, ' ' the eyes being on the tips of the second pair. 

 The lung-sac, besides serving as a breathing organ, also 

 enables the snail to rise or sink according as the animal 

 varies the size of the sac and consequently the amount of 

 air in it. All the Pulmonata are hermaphroditic, each 

 individual producing both sperm- and egg-cells. The 

 eggs of the pond-snail " are laid in gelatinous transparent 

 capsules, half an inch to an inch in length, flattened and 

 linear or oblong in outline. After a few snails have been 

 kept a short time in a small vessel of water with their 

 appropriate food, these egg-capsules may be looked for 

 on the bottom and sides of the vessel or closely adherent 



