INTRODUCTORY CHAPTER. 



9 



by a number of threads, which they spin and 

 fix to different parts of the rock or other sub- 

 stance on which they dwell. In the locomotive 

 Mollusca the foot is often serviceably employed 

 for the purpose of burrowing in the sand and 

 mud, to enable them to conceal themselves 

 underneath, being used as a scoop with much 

 effect, and thus enabling the animal to hide 

 itself in a short time, and to emerge when all 

 danger is over. 



With regard to the food on which Mollusca 

 live, there is as great a variety as exists among 

 animals of a more complicated organization: 

 some are carnivorous, or Zoophaga # , others her- 

 bivorous, or Phytophaga f , and the substances on 

 which they feed, whether animal or vegetable, 

 are consumed by them in various states, some 

 preferring the living, whilst others prefer the 

 dead or putrescent form. To meet these dif- 

 ferent forms of nutrition, various and diver- 

 sified organs are required; and it will be 

 found that in these animals, as well as in the 



* From zo-on, Gr., an animal, and phago, Gr., to eat, 

 f From phi/ton, Gr., a vegetable, and phago* 



