412 APPLIED BIOLOGY 



pods live in the sea and have in the respiratory chamber 

 feathery gills, which absorb oxygen from the water and give 

 off carbon dioxide. 



Very generally the hind-end of the foot of sea-snails bears 

 a hard plate which closes the mouth of the shell when the foot 

 is retracted. 



In most gasteropod shells the spiral is dextral ; that is, 

 starting at the apex and following the whorls as down a spiral 

 staircase one would continually turn to the right; but 

 some snails are sinistral (to the left). Two of our common 

 pond-snails (Planorbis and Physa) have this peculiarity. 

 The significance of the direction of the spiral is unknown. 



The covered shells of the garden-slugs are colorless. 

 Cowries are polished on the outside because folds of the 

 mantle cover and secrete pearly substance on the outside 

 when the animal is expanded (" out of its shell "). In 

 most gasteropods the shell is covered with an epidermis, but 

 this gets rubbed off as the shell grows old. 



Ridges and spines on univalve shells are common. Con- 

 tinuous ridges or lines extending spirally around the shell from 

 its mouth to apex are due to constant and similar ridges on 

 the edge of the secreting collar. Ridges or rows of spines 

 parallel with the mouth of the shell and the lines of growth 

 are secreted periodically (example, harp-shell). Sometimes 

 a wide lip at mouth of the shell is formed only when the 

 shell is full grown, and such a shell will have no such ridges as 

 in a harp-shell. 



341. The Highest Mollusks. The squids, octopus 

 (" devil-fish "), and nautilus represent the highest forms of 

 mollusks. All of them are marine. 



The common squid (Fig. 146) has a cylindrical body, 

 tapering to a point at the hind end. The movable head 

 has a group of ten arms surrounding the mouth, and the 

 arms have peculiar suckers adapted to holding prey. A 

 large eye is on either side of the head. 



