CLAilS, UYSTEK8 AND ilLSSELS 



121 



are rare. Oliver Wendell H(jliiiew has made ''The Cham- 

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



Characteristics and relationstiips of the mollusks. — All 

 of the members of this branch have soft bodies. With 

 the exception of the slugs, squids, euttlefi.shes, and a 

 few other.s, their bodies 

 are protected by shells 

 which are usualh' com- 

 posed of one or two 

 valves. The majority of 

 mollusks live in water 

 and breathe by gills. 

 Some breathe air directly 

 by means of lungs. 



It is thought that tlie 

 branch Mollusca is more 

 closely related to tht' 

 Annulata than to any of 

 the preceding branches. 

 It is certain that they 

 are the highest animals 

 so far .studieil. - " ' • 



Adaptations to environment. — Perhaps the first adapta- 

 tion to note is the uni([ue one which enables the long clam 

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

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

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

 air, water, and food. 



Although the scjuids liaA'e no out.side shell for protection, 

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

 water, blind the pursuer, and so escajie. 



The lung of the pond snail, Ijesides acting as a lung, serves 



