MOLLUSKS 323 



coast. The collector who has successfully taken a number of 

 these will not fail to note the varying degrees of width of the 

 girdle in the different species. In some 

 cases the girdle partially covers the over- 

 lapping shell plates. The largest of all 

 the chitons inhabits the California shores 

 and is known as Cryptochiton stelleri. 

 It measures fully six inches in length and 

 three in width. In Cryptochiton the gir- 

 dle is continued over the shell plates as a tough, gritty, brownish 

 covering. The shell plates are white, and, as detached pieces, are 

 frequently to be found cast up upon the California beaches, where, 

 from their curious shape, they are often referred to as " butterfly- 

 shells." 



On the east coast of America there are comparatively few 

 species, and these are all small and inconspicuous. The three 

 commonest ones are Chiton ruber, C. albtis, and C. apiculatus. 

 They may readily be found on stones or dead shells fished up 

 from below low-tide mark, or in rock-pools left by the receding 

 tide. 



ORDER APLACOPHORA 



This second order of the Amphineura is entirely destitute of 

 the shell plates characteristic of the first. The outer mantle sur- 

 face, however, develops a number of calcareous granules which 

 correspond to a shell. The Aplacopliora are symmetrical, having 

 the mouth and the excretory opening at the anterior and posterior 

 ends respectively. The foot is greatly modified, being narrowed 

 at times to a mere slit along the ventral surface. Some of them 

 have long, worm-like bodies. This is no doubt a very primitive 

 form of mollusk, or, if not a primitive form, it has greatly degen- 

 erated from some higher type. They are supposed to be com- 

 mensal in habit and live in the deeper water offshore. 



The Aplacopliora are not littoral animals, and it is extremely 

 unlikely that one will ever encounter them upon the beach, un- 

 less, possibly, after the severest of storms. We may therefore 

 leave them with this mere mention and proceed to the second 

 class of niollusks, the Gasteropoda. 



