324 NATURAL HISTORY. 
is Cuttle-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 XXXII. 
MOLLUSKS — continued. 
557. Tur class of Gasteropoda is mostly composed of 
Mollusks that live in a wntvalve 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. 255.—Limnea Stagnalis, 
