98 THE MOLLUSCA 



a number of small retractile feelers in place of the powerful, sucker- 

 clad arms. A vertical section through the shell shows that it 

 is divided into a series of chambers connected to each other by 

 a slender tube or siphon, the exact function of which is still doubt- 

 ful. The partitions across the shell represent periods of growth : 

 the Nautilus, as it outgrows the capacity of the outer chamber 

 in which it lives, forms a new one of larger size, and separates 

 it from the old chamber by a transverse partition. 



The order Gastropoda is a very extensive one, and includes 

 a vast number of species inhabiting the sea, fresh water, and 

 the land ; while many occupy a transitory position between salt 

 and fresh water and water and land, and are found in salt marshes 

 in brackish water, in swamps and mud-flats, on the margins of 

 the sea, rivers, and lakes, and high up on rocks where they are 

 only occasionally covered by the tide or moistened by the spray. 



The Gastropod Molluscs are distinguished by the broad, flat 

 foot which occupies the whole of the under surface of the body. 

 On this strange foot the animal creeps or glides along, with a slow, 

 rhythmical movement, by the expansion and contraction of strong 

 muscles. If a Periwinkle is watched as it moves over the side of 

 a glass tank, or a snail as it crawls on a window-pane, the muscular 

 movements of the foot may be seen following one another in rapid, 

 wave-like succession. 



The animals possess a more or less distinct head, a jaw or a 

 pair of jaws, and a ribbon-like tongue covered with rows of minute 

 teeth called the radula, lingual ribbon, or odontophore. The length 

 and breadth of the radula varies considerably in different genera ; 

 in the air-breathing Molluscs it is very broad and rather short, 

 but in the Littorina, dwellers in the littoral zone between high- and 

 low-water mark, it is very long and narrow, and when not in use 

 is kept coiled Up like a Watch-spring at the back of the animal's 

 throat. 



The teeth on the radula vary greatly in form and number. 

 The carnivorous Molluscs have, as a rule, comparatively few teeth, 

 which are large and powerful, usually hooked, sickle-shaped, or 

 barbed like arrow-heads, and sometimes provided with poison 

 glands. Vegetable feeders, on the other hand, have, normally, 

 quantities of minute teeth which may be long and slender or 

 blunt and rounded. The tongue of the Limpet is often longer 



