MOLLUSCA : GENERAL CHARACTERS. 297 



others; or longitudinal in the middle, and transverse in the 

 outer and inner strata. 



All living shells have an outer layer of animal matter, which 

 is known as the "epidermis," or " periostracum." This is 

 sometimes of extreme tenuity, 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 par- 

 titioning 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 condition 

 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 " re- 

 versed," or " sinistral." The reversed 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. 



