MOLLUSKS. 333 



lusks is used. Some, thrusting it out, attach it to some 

 support, and then, by contracting it, pull themselves 

 along. Some use it to push themselves forward, as a 

 man in a boat pushes himself from the shore with his oar. 

 And some, by bending the foot and then quickly straight- 

 ening it, leap forward. There is a little Mollusk, the 

 Tfinthina, or Oceanic Snail, Fig. 264, which has attached 



Fig. 264. lanthina with its raft. 



to its foot a raft of singular construction. It is made of 

 numberless vesicles, , filled with air. Its purpose is to 

 float the eggs, b. You see at c the gills of this little an- 

 imal, and at d its tentacles and eye-stalks. The lanthina 

 is often met with in great numbers in companies in the 

 open sea. In rough weather much damage is often done 

 to their beautiful floats, and sometimes they are wholly 

 destroyed. 



574. The Tunicata form an aberrant group of the Mol- 

 lusca, verging, in their organization, toward the Radiata, 

 the only remaining sub-kingdom to be noticed. Although 

 it would be interesting to consider this group, I shall pass 

 it by. I shall also omit another class, the Polyzoa, for- 

 merly supposed to belong to the Radiates, but recently 

 ascertained to belong to the Mollusks. 



Questions. In what do the Gasteropods mostly live ? What is said 

 of the form of their shells ? What is said of the naked Gasteropods ? 

 Describe the structure of these animals. What is said of their teeth ? 

 Describe the structure of the Limpet's tongue. What are the terres- 

 trial Gasteropods ? Describe the common Slug. Describe the com- 

 mon Snail. What is said of the Pond Snails ? What are included 

 in the order of Gasteropods called Palmonifera ? What is said of the 



