CLAMS, OYSTERS AND MUSSELS 



121 



are rare. Oliver Wendell Holmes has made "The Cham- 

 bered Nautilus" the subject of one of his finest short poems. 

 Characteristics and relationships of the mollusks. — All 



of the members of this branch have soft bodies. With 

 the exception of the slugs, squids, cuttlefishes, and a 

 few others, their bodies 

 are protected by shells 

 which are usually com- 

 posed of one or two 

 valves. The majority of 

 mollusks hve in water 

 and breathe by gills. 

 Some breathe air directly 

 by means of lungs. 



It is thought that the 

 branch MoUusca is more 

 closely related to the 

 Annulata than to any of 

 the preceding branches. 

 It is certain that they 

 are the highest animals 

 so far studied . ^'''°- ^^- ^ ^'"'^ °'' p^'^''^ ''''^*"""- 



Adaptations to environment. — Perhaps the first adapta- 

 tion to note is the unique one which enables the long clam 

 to live so deep in the mud, out of reach of its enemies above. 

 There it lies six or ten inches deep in the mud, and yet, by 

 means of its long siphons, is able to get a supply of fresh 

 air, water, and food. 



Although the squids have no outside shell for protection, 

 they have a sac full of pigment by which they can color the 

 water, blind the pursuer, and so escape. 



The kmg of the pond snail, besides acting as a Imig, serves 



