2,44 ELEMENTARY ZOOLOGY 



fish by the lower hook-like projections of the valves and 

 leads a truly parasitic life for two months, after which it 

 undergoes a metamorphosis and falls to the bottom again, 

 there to begin an independent existence. Mussels often 

 congregate in favorite mud or sand banks. Their food 

 consists primarily of small organisms, both plants and 

 animals, which are taken from the water entering the 

 mantle-cavity. Mussels move about slowly over the 

 muddy bottom of the stream by means of the muscular 

 foot. 



OTHER MOLLUSCS. 



The branch Mollusca includes the fresh-water mussels, 

 the clams, oysters, snails, and slugs, the cuttlefishes, and 

 all that host of animals we call ' ' shells ' ' or shell-fish, which 

 we know familiarly only by the shell which they make, 

 live in, and leave at death to tell the tale of their exist- 

 ence. Not all the molluscs, however, form shells, that 

 is, external shells which serve as houses. The familiar 

 slugs do not, nor do a number of ocean forms called 

 nudibranchs, which are somewhat like the land-slugs, only 

 much prettier and more attractive. All the cuttlefishes 

 and octopi are also without the hard calcareous shell. 

 But most of the molluscs are shell-bearing animals. The 

 shell may be bivalved, as in the mussel and clam, or uni- 

 valved, that is, composed of a single piece which may be 

 spirally twisted, as with the snail, or otherwise curiously 

 shaped. The variety in the form, colors, and markings 

 of the shells indicates the great diversity among molluscs. 

 Molluscs live on land, in fresh water and in the ocean. 

 No depths of the ocean abysses are too great for the 

 octopi, no coast but has its many shells, hardly a pond 

 or stream is without its mussels and pond-snails, and in 

 all regions the land-snails and slugs abound. 



