MOLLTJSCA: GENEEAL CHARACTEES. 231 



normal condition of the species, or it may occur simply as a 

 variety of a form, which is normally right-handed, or ' dextral.' 

 The sub-kingdom Mollusca is divided into two great divi- 

 sions, termed respectively the Molluscoida, and the Mollusca 

 proper. In the former of these the nervous system consists of 

 a single ganglion or principal pair of ganglia, and there is 

 either no circulatory organ or an imperfect heart. In the 

 latter the nervous system consists of three principal pairs of 

 ganglia, and there is a well developed heart, consisting of at 

 least two chambers. 



