MOLLUSCA. 273 



When the columella is hollow (perforated), the end is 

 called the " umbilicus." When the whorls are coiled 

 around the axis in the same plane, we have a discoidal 

 shell, as the Planorbis. The mouth, or "aperture," of 

 the shell is " entire " in most vegetable-feeding Snails, and 

 notched or produced into a canal for the siphons in the 

 carnivorous species. The former are generally land and 

 fresh-water forms, and the latter all marine. In some 

 Gasteropods, as the River -snails and most Sea -snails, a 

 horny or calcareous plate (operculum) is secreted on the 

 foot, which closes the aperture when the animal with- 

 draws into its shell. In locomotion, the shell is carried 

 with the apex directed backward. 



The body of most Gasteropods is unsymmetrical, the 

 organs not being in pairs, but single, and on one side, 

 instead of central. The mantle is continuous around the 

 body, not bilobed, as in Lamellibranchs. A few, as the 

 common Garden-snail, have a lung ; but the vast majority 

 breathe by gills. The head is more or less distinct, and 

 provided with two tentacles, with auditory sacs at their 

 bases ; two eyes, which are often on stalks ; and a strap- 

 like tongue covered with minute teeth. The heart is sit- 

 uated, in the majority, on the right side of the back, and 

 has two cavities. The nervous ganglia are united into an 

 oesophageal ring or collar (Figs. 45, 154). All, except the 

 Pteropods, move by means of a ventral disk or foot. 



Gasteropods are now the reigning Mollusks, comprising 

 three fourths of all the living species, and are the types 

 of the subkingdom. They have an extraordinary range 

 in latitude, altitude, and depth. 



Omitting a few rare and aberrant forms, we may sepa- 

 rate the class into the following orders : 



1. Pteropods. These are small, marine, floating Mol- 

 lusks, whose main organs of motion resemble a pair of 

 wings or fins coming out of the neck, whence the com- 



18 



