102 :>;OG,LOGY. 



edges, and is often of a spiral form, not unlike a small 

 nautilus. Some species are destitute of a shell. 



137. Digestion. There usually is no distinct line of 

 separation between the head and the body. The mouth 

 is surrounded by tentacles, and has an odontophore. 

 There is a stomach, liver, and intestine, with anal open- 

 ing, on the ventral surface of the body. They are said to 

 subsist on small Crustacea, &c. 



138. Circulation. There is a heart with two cavities, 

 an auricle, and a ventricle. 



139. Respiration is effected by a portion of the sur- 

 face which is covered with cilia. This is sometimes 

 placed in a chamber within the mantle, and is sometimes 

 external. 



140. The Nervous System consists of three pairs of 

 ganglia. The principal mass is below the oesophagus. 



141. Development. All the Pteropoda are hermaphro- 

 dite, male and female organs being united in each indi- 

 vidual. The young undergo metamorphosis. 



CLASS 3. Gasteropoda. 



142. The Gasteropoda (dr.gaster, the belly; 

 is a very large class. It includes land-snails, sea-snails, 

 fresh-water snails, slugs, limpets, whelks, &c. It is so 

 called, because the organ of locomotion generally consists 

 of a flat muscular disc occupying the lower part of the 

 body, which is termed the " foot." By the alternate con- 

 traction and expansion of this organ they glide slowly 

 along. In the Heteropoda (Gr. heteros, diverse; poda, feet), 

 the foot is furnished with fin-like expansions, which adapt 

 the animal for swimming. The hinder part of the foot, 

 in many of the Gasteropoda, is provided with a horny or 

 calcareous plate, called the operculum (Lat. operculum, a 

 lid). When the animal retreats within the shell, which 

 it is enabled to do by the retractor muscle, this operculum 

 closes up the aperture. 



