MOLLUSCS, OR SHELL-PISH. 229 



We cannot here go into the minute details of the struc- 

 ture of Chiton, but may briefly call attention to its more 

 salient features. It is a true Mollusc; it has a mantle 

 which secretes the dorsal shells, a ventral foot, it breathes 

 by gills. It resembles the Gasteropods in the condition of 

 the foot, and in possessing a radula or tooth-ribbon within 

 the mouth. But it differs in many respects from the 

 Gasteropods. Instead of having one shell it has eight; 

 in place of the single gill of most Gasteropods it has eight 

 pairs ; instead of being unsymmetrical, with the organs 

 apparently twisted to the right, it is perfectly symmetrical 

 with mouth at one end and anus at the other, like worm 

 or Arthropod. In brief, it is a simple and primitive form. 

 It should be especially noticed that it resembles worms and 

 Arthropods in showing traces of segmentation. We have 

 already noticed that in both these groups the body is made 

 up of a repetition of similar parts is distinctly segmented. 

 Now in the Mollusca such segmentation is typically absent, 

 its absence being one great point of contrast with the 

 Arthropods. The number of shells in Chiton, and their 

 relation to the gills, point, however, to the existence of 

 segmentation in this primitive form. This is a point of 

 much interest to those who care about problems of origin. 



The species of Chiton are chiefly distinguished by the 

 minute characters of the shells. The commonest form is 

 Chiton marginatus, and is very variable in colour and size, 

 but is distinguished by the finely granular surface of the 

 valves. Each valve has a slight central keel prolonged 

 posteriorly into a small beak, and is divided into three 

 areas by two diverging lines. All the areas are similarly 

 marked with fine dots, sometimes partially rubbed off in 

 old specimens. The colour is usually greenish, marked 

 and dotted with pale colour, but bright red varieties also 

 occur. 



Another common and much prettier species is 0. fascicu- 

 laris, characterised by its comparatively small valves, and by 

 groups of bristles placed on the margin of the mantle. 

 There are eighteen of these groups, four being placed in 

 front of the first valve and a pair in front of each succeed- 

 ing valve. The individual valves should be examined with 

 a good lens, when their brilliant colouring and beautiful 



