370 MANUAL OF ZOOLOGY. 



uity, but is sometimes very thick, the latter being especially the case with 

 those shells which are found in fresh water. 



In many of the spiral univalves, as the animal grows it withdraws itself 

 from the upper portion of the shell, often partitioning off the space thus 

 left vacant. In many instances the portion thus abandoned falls off, and the 

 shell becomes "truncated," or " decollated ; " this being the normal con- 

 dition in fully-grown examples of some shells. 



In the great majority of univalves the shell is coiled into a spiral, the 

 direction of which is right-handed, but in some cases the spiral is left- 

 handed, and the shell is said to be "reversed," or "sinistral." There- 

 versed shell may occur as the 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. 



