690 ON MOLLUSCS IN GENERAL. 
Some molluses 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 cesophagus. The mouth conducts immediately 
into the cesophagus, 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. 
