MOI I i , 



239 



usual veliger type. The sea butterflies, or Pteropoda, are 

 minute pelagic animals, very transparent, and rarely attaining a 

 length of two or three centimeters. The foot is at the ante- 

 rior end of the body, and is flattened out in two Lateral tolds 

 or winglike structures, by means of which the animal swims 

 (Fig. -45). In some a delicate, transparent shell is present, in 

 others there is no shell. 



FIG. 245. Various shell-bearing Pteropoda. / fin; /, liver; 0, ovary; sk, shell. (After 

 Souleyet, from Parker and Haswell's Manual.) 



Order 3. Pulmonata 



The Pulmonata (Lat. pulmo, lung) are terrestrial or fresh- 

 water Gastropoda without gills, and having the mantle cavity 

 modified into a sort of lung or respiratory chamber. Those 

 species that live in the water must necessarily come to the sur- 

 face occasionally to change the air in their lungs ; but some 

 living at the bottom of deep lakes have the chamber filled with 

 water, and thus use it as they would gills. The edge of the 

 mantle is fused with the body wall except at one point, usually 

 on the right side of the body, which serves as the respiratory 

 pore or opening into the mantle cavity. Some have a large 

 shell, usually dextral, others a very rudimentary shell, and 

 some no shell at all. There is no operculum to close the 

 shell as in the Prosobranchia ; but during the dry season 

 or at the approach of winter the animal secretes a temporary 

 parchment-like membrane which protects it from desiccation. 

 The mouth is provided with a radula as in many other Mollusca. 

 All the Pulmonata are hermaphroditic, and a few species are 

 viviparous. 



