THE SHELL-ANIMALS 411 



(D) Internal organs. The form of the body must be studied in 

 museum specimens prepared by special methods, and preserved in 

 formalin. The breathing pore leads into the lung-sac. The pos- 

 terior end of the digestive canal is near the breathing pore, the canal 

 having been twisted so as to bring its opening near the edge of the 

 shell. Heart, liver, kidney, loop of intestine, and reproductive 

 glands make up most of the mass within the shell. Note that this 

 mass of organs has the form of the shell, and that the upper whorls 

 are filled even when the snail's foot 

 is out of the shell. The animal is 

 hermaphroditic (283). The nerv- 

 ous system consists of masses of 

 nerve-cells (ganglia), chiefly placed 

 anteriorly in head and foot, and FlG - 145 - Garden slug (Limax). 

 with nerves from these ganglia to ' *f ntacles : "V mantle covering 

 various parts of the body. Careful 

 study of the internal organs of a 

 snail is usually made in college courses of zoology, but we have 

 not time in this course. 



(L) Study of empty snail-shell. The small end is the apex, 

 the central axis is the columella, and the opening is the mouth of the 

 shell. Notice direction of the spiral as compared with threads on 

 an ordinary wood-screw. At a broken edge may be seen an inner 

 pearly layer, a middle thick layer, and a thin outer layer. The 

 middle and outer layers are secreted by the collar, while the whole 

 inner skin (which is the mantle) secretes pearly lining. Examine a 

 shell from which one side has been chipped away with strong forceps 

 so as to expose the interior. 



Garden-slugs, which are usually to be found in gardens and green- 

 houses, are similar to snails, except that the shell is small and covered 

 with skin. Obviously, such a degenerate shell is of little use as a 

 protecting organ. 



340. Snails and their Allies : Gasteropods. The garden- 

 snails are members of a group of mollusks known as gastero- 

 pods (class Gasteropoda). The name means stomach in the 

 foot. Since they have a shell consisting of one piece, 

 they are often called univalves to distinguish them from the 

 bi-valved clams. 



While the land-snails and their near relatives, the slugs, 

 breathe air by means of a simple lung-chamber, most gastero- 



