MOLLUSCS. 



83 



Fig. 77. — A very 

 young tinkle-shell 

 {Anomia), showing 

 the large notch 

 made by the byssus. 



forms. In the tinkle-shell it is a 'byssus' (a structure exactly comparable 

 with the byssus of the mussel, soon to be described). This at first passes 

 out from the foot to the point of attachment between the 

 valves ; but as these grow, a notch is left for the byssus, 

 which gradually becomes deeper, and at last is converted 

 into a hole, through which the byssus passes. Our figure 

 shows the young tinkle-shell at about the middle of the 

 process. The scientific name, Anomia, refers to the irreg- 

 ularity in the growth of these forms, while the common 

 name, tinkle-shell, refers to the musical note produced 

 when several are shaken together ; the other two terms 

 refer to the colors of the dead shells. The living animals occur most 

 abundantly attached to rocks or to other shells. 



The scallops (JPecten), of which there are some two hundred species 

 now living, are relatives of the oysters. The shells are flattened, and the 

 hinge line is nearly straight, while the rest of the outline is circular, the 

 surface being coarsely or finely ribbed, and sometimes beautifully colored. 

 One species which is abundant on our coast from Cape Cod to Florida is 

 extensively used as food, some esteeming it highly, while others do not 

 relish its sweetish taste. In England two species are eaten, 

 one being called scallop ; the other, quin. 



The scallops are noticeable from the fact that they 

 possess well-developed eyes. Were these placed near the 

 mouth, they would be of but little use, for they would be 

 too far within the shell ; but seated on the edge of the 

 mantle, just within the margin of the shell, they are where 

 they are of the most use. Other bivalve molluscs, like the 

 thorny oysters, have eyes in the same position ; but to those 

 which, like the clam (Myd), live buried in the mud or sand, 

 eyes in a corresponding position would not be of any advan- 

 tage to the animal. In these forms the siphon is the most 

 exposed portion, and here the eyes are situated. 



One must not think that these eyes are highly complex 

 organs. They are reallv very simple. All that is necessary fiq. 78.— Portion of 



J " J - 1 ^ the integument of 



to form an eye is a refractive body, pigment to absorb the the quahog, great- 



•' J 7 r ° ly magnified, 



light, and nerves to carry the sensation to the central ner- showing the 



J patches of pig- 



vous system. In the clam and oyster these parts are pres- ment which tope*. 



J *- - 1 ceptive of light. 



ent. In the proper places one can distinguish with the 

 naked eye slight patches of dark pigment, and, when examined under the 

 higher powers of the microscope, is seen to be made of cells covered with 

 a refractive cuticle, and containing a deposit of pigment in their outer 



