690 ON MOLLUSCS IN GENERAL. 



Some molluscs have no head distinct from the body. Here the 

 principal mass of the nervous system is not placed near to or above 

 the mouth, but if nervous ganglia be found in this situation, they 

 are inferior in development and size to other nervous ganglia; 

 no nerves for special organs of sense arise from the nervous mass 

 situated above the oesophagus. The mouth conducts immediately 

 into the oesophagus, and has no teeth or tongue. These are named 

 headless molluscs (Acephala). The other molluscs have a head more 

 or less clearly distinct. The head-bearing molluscs (Cephalophora 

 or mollusca encephala) are more perfectly organised, and amongst 

 them alone are species found which live on land ; most of the 

 species, however, reside in the water, like the acephalous molluscs. 

 These last may be arranged in two classes, the naked and the 

 bivalve acephalous molluscs. 



