MOLLUSCOIDA. 233 



"arms," which are usually coiled round in a spiral, 

 and sometimes supported by a spiral skeleton, con- 

 sisting of loops derived from the dorsal valve of the 

 shell to which they are fixed. They bear ciliated ten- 

 tacles, and have been compared with the tufts of the 

 worm Sahella ; they are partly respiratory, and partly 

 used for sending food particles to the mouth. There 

 are numerous fossil forms, the class having attained 

 its maximum in past periods. 



rig. 82.— One of the surviving species of Lamp-shell (Tereliratiia 

 JUaajiUata) ; shell seen in profile. 



There are some forms in which the so-called arms 

 are not fixed, but are capable of being extended to 

 some distance. In this case the supporting structures 

 are short. 



