324 NATUEAL HISTOKY. 



is Cnttle-fish bone ? What is said of the Argonaut ? What of the 

 Pearly Nautilus ? What is said of the Pteropod group ? What of 

 the Clio Borealis ? 



CHAPTER XXXin. 



MOLLUSKS — continued. 



557. The class of Gasteropoda is mostly composed of 

 Mollusks that live in a univalve shell, which is usually of 

 a spiral shape. You have two different forms of the 

 spiral in Fig. 249, page 317. Some of the species, as the 

 Slug, are naked or destitute of shell. There is, however, 

 in these, sometimes a small shell, generally imbedded in 

 the mantle, just over the cavity which contains the lungs. 

 The body of the Gasteropods is terminated in front with 

 more or less of a head, having fleshy tentacula, varying 

 from two to six in number. The back is covered with a 

 mantle which secretes the shell. On the under side of 

 the animal is the fleshy mass called the foot. In those 

 which have a shell, all the body remains in it except the 

 head and the foot. These project beyond it when the 

 animal expands them for walking, but they can be with- 

 drawn into the first turns of the shell at pleasure. In 

 most of the aquatic Gasteropods there is on the foot a 

 plate of horny substance, which shuts over the opening 



Fig. 25S — LlmnKa Stagnalis. 



